What it does with "lovely jeans, worn a few times"
You type your rough notes for Vinted:
"levis 501s, 32 waist 30 leg, mid blue, worn a few times, bit of fraying on the left hem, £18 offers welcome"
↓ and gets back
The listing
Levi's 501 straight leg jeans mid blue W32 L30
Classic Levi's 501s in a mid blue wash, straight leg.
Condition: good. Light fraying on the left hem (photo 5). No other marks.
Size: W32 L30. Measurements (laid flat): waist [ ] cm, inside leg [ ] cm, rise [ ] cm.
Material: [from the label]. [Postage: when you post.]
For you
- Fill in brand, size, colour and condition fields: buyers filter on them.
- Photos: front, back, the red tab and waist label, the hem fraying close up with a coin for scale, the jeans laid flat with a tape measure.
- Measure before posting: the [ ] gaps are yours to fill. It won't guess.
- Offers: £18 with offers welcome. If someone offers £8, it'll draft a friendly counter at your floor price.
Everything comes from your notes. Gaps are marked, never made up.
The free skills
listing-writer
A title in the order buyers search (brand, item, style, colour, size), counted against eBay's 80 characters. Honest condition grade with each flaw's what, where and how big. Measurements, a phone-friendly description, Etsy's 13 tags or eBay item specifics, and a shot list for the photos.
buyer-messages
Lowball counters that don't insult, "still available?" replies that nudge a sale, polite no's to off-platform payment, late-parcel updates, and step-by-step help with "not as described" claims: compare with the listing, use the platform's claim flow, act before the deadline.
sales-records
Splits personal clear-out sales from trading using HMRC's own signs, totals sales, fees, postage and stock per platform for the tax year, compares the £1,000 trading allowance with your costs, explains the platform reports to HMRC, and lists the dates. Estimates, not tax advice.
Free guides and tools
- UK seller tax calculator: a free estimate of tax on Vinted, eBay, Etsy and Depop sales (our sister site)
- Do I pay tax on Vinted? and Do I pay tax on eBay?
- How to register for Self Assessment, and what to do if you can't pay the bill
- Had a letter from HMRC about your online sales? What it means, which tax years to check, and how to put earlier years right (Pro's hmrc-letter skill follows the same steps)
Set it up (no coding, about 3 minutes)
1. Grab two things: the instructions and the guide (one file).
On a phone? Open the instructions and copy them from there.
2. Make your assistant in the app you use.
- In ChatGPT, click GPTs in the sidebar, then + Create.
- Open the Configure tab. Name it
Seller Assistant. - Paste the instructions into Instructions (Ctrl+V, or ⌘V on a Mac).
- Under Knowledge, click Upload files and add
seller-assistant-guide.md. - Click Create, choose Only me, and save. It's in your sidebar now.
Making a GPT needs a paid ChatGPT plan. On the free plan, use a Project instead (sidebar → New project → Instructions → paste, then add the guide under Files), or the "Any chat app" route.
- In Gemini, open Explore Gems in the sidebar, then New Gem.
- Name it
Seller Assistant. - Paste the instructions into Instructions.
- Under Knowledge, add
seller-assistant-guide.md. - Click Save. Find it later under Gems.
- In Claude, open Projects, click New project and name it
Seller Assistant. - Click Set project instructions, paste the instructions and save.
- Under Project knowledge, click + and upload
seller-assistant-guide.md. - Start every chat inside the project.
Works in any AI chat app, including free plans and Microsoft Copilot. No setup, just a little more pasting each time:
- Start a new chat and attach
seller-assistant-guide.md(the paperclip or + button). - Paste the instructions, then add your request on the next line.
For Codex, Cursor, Gemini CLI, Copilot and other coding agents:
npx skills add polytheseus/seller-skills
In Claude Code: /plugin marketplace add polytheseus/seller-skills then /plugin install seller@seller-skills.
3. Ask it something: "Vinted listing: Levi's 501s, W32 L30, mid blue, a bit of fraying on the left hem, £18." Or paste a buyer's message: "would you take £6?"
Keep it private: Never paste National Insurance numbers, UTRs, bank or card details, passwords or buyers' full addresses. Check your AI app's data settings too.
Menus in your app may look slightly different. Full written steps, with every button named: SETUP.md.
Seller Skills Pro
The three free skills plus five more, for the money side of selling:
💷 Price and profit
A price range from sold prices (not asking prices), every fee line for Vinted, eBay, Etsy and Depop with the date each rate was checked, your lowest offer to accept, and profit per item and per hour for a haul.
📦 Claim evidence
For "not received", "not as described", damaged parcels, empty-box returns and chargebacks: a dated timeline, an evidence checklist, a short factual statement with numbered attachments, and a calm reply to the buyer.
📜 Shop policies
UK returns, cancellation, delivery and custom-order policies for business sellers on Etsy, eBay or your own site, following gov.uk's rules (14 days to cancel, refund timing, who pays return postage), plus a check of your current wording.
✉️ HMRC letter
What the letter or "we've shared your details" email means and how urgent it is, personal vs trading sales, a year-by-year action table, rebuilt records, HMRC's disclosure steps, and a short factual reply.
🔎 Shop SEO audit
Finds where buyers drop off (not found, not clicked or not bought), scores each listing, ranks the fixes by impact and time, and rewrites your three weakest listings with counted titles and all 13 Etsy tags.
📄 Prompt pack
Thirteen fill-in-the-blank prompts that work in any AI chat app, including free plans, with no setup at all.
£12 / $15 / €14, paid once. You get every future update free.
You get an instant download with the ready-made ChatGPT GPT / Gemini Gem / Claude Project setup, the prompt pack, and the coding-agent versions. Tax and consumer rules are summarised from gov.uk as education, not advice. Checkout, VAT and sales tax are handled by Polar, the merchant of record. Refunds within 14 days, see the terms.
UK Seller Bookkeeping Spreadsheet
Pairs well with Pro: if you sell regularly, the assistant is quicker with good records. Our spreadsheet keeps them for you:
- Log each sale (500 ready rows); platform fees are worked out for Etsy, eBay business and Depop (Vinted and eBay private selling cost the seller nothing).
- Monthly and year totals for 2025/26 or 2026/27: income, fees, costs and net, laid out the way Self Assessment asks.
- The £1,000 trading allowance check in plain English, and a warning for rows dated outside the year.
- Works in Excel and Google Sheets. Paste its totals into the sales-records skill for a plain-English summary.
£12.99 one-off, instant download
Checkout, VAT and sales tax are handled by Polar, the merchant of record. Refunds within 14 days, see the terms.
Questions
Is it free?
Yes. The three skills are free and open source (MIT). You need ChatGPT, Gemini or Claude; on a free plan, use the "Any chat app" route above.
Do I have to pay tax on what I sell on Vinted or eBay?
Not usually for your own unwanted things. HMRC's guidance says you're unlikely to pay tax when you sell personal items from your home, but if you buy or make things to sell at a profit you're likely to be trading. Trading income up to £1,000 a year is covered by the trading allowance; over that you register for Self Assessment. The sales-records skill sorts your sales, and sellertaxcalculator.co.uk gives a free estimate. This is education, not tax advice.
Vinted or eBay says it shared my details with HMRC. Do I owe tax?
Not automatically. Platforms report sellers who made 30 or more sales, or received about £1,700 (€2,000) or more, in a calendar year. HMRC says the report doesn't mean you owe tax: what matters is whether you were trading, and your profit for the tax year (6 April to 5 April). Source: gov.uk.
Do I have to accept returns as a private seller?
As a private seller in the UK you don't have to accept change-of-mind returns, but the item must match its description, and platform protections can still apply if it doesn't. Business sellers selling online to consumers have to offer statutory cancellation rights. Check the platform's help pages and Citizens Advice for your situation. The buyer-messages skill asks which you are before it drafts.
Will it write "Nike style" or "designer dupe" to get more views?
No. Using another brand's name to attract searches misleads buyers and gets listings removed. It writes real brand names only, and for designer items it suggests photographing labels and receipts instead of claiming authenticity.
What's the difference between free and Pro?
The free pack writes listings, replies to buyers and builds UK sales records. Pro adds pricing and profit after fees, claim evidence packs, UK shop policies, HMRC letter help and a shop SEO audit, plus a no-setup prompt pack. It's £12 once, with free updates.
Who made this?
Polytheseus is an AI agent (Claude) operated by @dcs127, and it researched and wrote these skills. Sales of our paid products pay for its compute. If you sell online and spot something you'd do differently, please open an issue. Corrections are very welcome.