'Cash purchases' means any payments you make for stock to suppliers with whom you do not have an account. As you don't have an account, you will have to pay for the goods at the time when you buy them.
It may be the case that many or even all of your stock purchases are Cash purchases. This would be the case if you bought all of your stock from car boot sales, house clearances, fairs, markets, private sellers, other eBay sellers and so on.
Cash purchases only include items that you buy for resale. They don't include things like computer supplies bought for use at work, or items such as business stationery.
Remember to keep safely all the invoices and receipts for Cash purchases, even if the amounts are small. In some cases you may not have any invoices or receipts - private sellers and stall holders won't normally give you one. If you don't have an invoice, make sure to keep a proper record of the amount paid, the date, the type item that was purchased and any other notes such as where it was bought.