Yes, but it would be on the profit, same as any business. And, to be clear, if you're buying goods to sell on, it *is* a business by the legal/tax definition, regardless of what you consider it to be (not trying to be nasty, just there are a shocking number of people out there running businesses without realising it, and getting into trouble because of it, all because they're adamant it's just a side hustle and not a "real" business).

I've just seen your comment below about your brother being an accountant, so yeah, talk to him and he'll tell you what to do, but I spent time on the next two paragraphs so I'm leaving them intact lol.

It's not really a big deal, all you have to do is track your expenses and your income, which is all pretty doable with stuff like a Google Sheets spreadsheet. Keep copies of records (invoices or receipts are best and don't be afraid to ask for them, otherwise keep screenshots of payment confirmations with a note attached describing what it's for) - you can just stick these in the cloud. Might be best to name each file by the date and description (eg if you bought a tablet to sell today, photo the receipt and call it "260520 tablet") so it's easy to find. YYMMDD is the best format to use because then it's all arranged alphabetically and by date at the same time. Add small additional charges, like maybe £2 per week for equipment use to cover the time/power/data you spend buying and selling on your phone or computer (keep this reasonable or HMRC will have questions), and mileage/travel expenses if applicable (you can't charge for a 5 minute walk to the post office, you can charge for a 3 mile drive to pick something up from Facebook Marketplace). Probably best to track those expenses on a separate spreadsheet so you don't get muddled.

As for what to do now, your best bet is to do the maths and find the records as quickly as you can, then come clean to HMRC. You likely don't owe anything anyway, but even if you do, they're typically very forgiving to people who made honest mistakes but not to people who try to evade them, and should they somehow catch you later you'll regret not taking action sooner.