If you're not doing it in the course of a business, you should be fine. After all, you can sell your car on eBay, and you don't get a tax demand for having done so.
If you start selling loads of stuff on eBay, HMRC might consider that you're running an unregistered business, and at that point you'd have to registered it as a self-employed business or set up a company or whatever and pay tax on the income.