If you are selling as a business i.e. making a profit on purpose, then you need to keep your own records. eBay's monthly invoices will tell you your gross sales, eBay fees and any postage labels purchased via eBay, but that's only half of the information you need for HMRC. You also need to keep a record of all your costs outside of eBay e.g. cost of items purchased to resell, spare parts, sundries (cleaning fluids etc.), packaging (if you pay for it), any items written off/losses outside of eBay. You need to know the date of each one, then decide what accounting year you are going to use (tax year, calendar year etc.) so you can total up the sales and costs by year.
As a sole trader, when you submit your tax return to HMRC they only require for your total gross sales, and the total of all your costs for the year. You don't have to provide a breakdown. However, HMRC are allowed to audit you and then they would presumably want to see evidence of where your totals came from.
The tax reporting is not difficult as long as you get in the habit of keeping good records as you go along.
