Not being in the UK I can't answer that with 100 percent accuracy, but I thiunk you'll find it true most places.
They'll usually accept whatever you submit if it looks correct. A printout from an accounting package should do it. They are going to have certain guidelines to work with, as far as what percent of sales were profit, what percent were expenses, etc. Usually, only if something there suggests they look further will they ask you to produce actual records.
That is when you need some PROOF. Actually you should be doing one thing automatically. You should have a seperate bank account for your business. You should deposit every single penny you take in as sales in that account. And you should pay all expenses from that account, as well as any withdrawals to yourself for personal spending.
If you do that, along with the accounting program, you can show that what you claim as income and what you actually received are the same. They may ask for backup proof. I can't imagine why they would not accpet ebay receipts and paypal or merchant account transaction records, that's how you earn your income. What other records are tehre?
Hand written anything are only useful if they are pre-numbered and there are no missing numbers. They can be standard salesbooks and recepts bought at an office supply store, but they should have consecutive numbers and there should be none missing. Bound books are best, since it is hard to lose a copy. This includes voided forms. You should keep them aslong as the laws require it, just like good forms. (US is 7 years).
Frankly, if you are trading on ebay there isn't much they can expect except the ebay receipts.