As the previous poster said, if you want to set all that stock off against your income you'll need to have all the receipts, just in case HMRC want to see them. It's unlikely, but they may ask for them at some point in the future.
Secondly, if they were part of a job lot, the simplest way would be to just average out the cost on each receipt, so all items hold the same value regardless of the actual value. For example, if you have a receipt for £1000 and there are 100 items on that receipt, then you value each one at £10.
You can earn up to £1000 per year without needing to file accounts, but you will need to let HMRC know you are doing this. If you do this though, you can't claim any expenses.
On the other hand, if you intend to turn this into a business and earn more than £1000 a year, then just start selling. You have up to 3 months from starting in which to register your business with HMRC. Just remember to keep all your records from the start..
I used to be an ebay/Amazon seller myself, so I do have some experience in this, but it's probably best to talk to an accountant, as things are constantly changing.