Item Recovery. Receipts.
You contact support who can fully verify all purchases
When you buy gems, you receive an email with details of your purchase. If you pay with paypal, you’ll also have your paypal statement as proof. Lastly, as Randulf said, CS will be able to access transaction history if you contact them.
The only “receipt” you receive for buying items on the gem store is the list on the right hand side of the gem store tab. It shows a list of your most recent purchases. But, again, CS would be able to see what you bought. You may have to provide screenshots or something to show proof of an item being missing, and you’d have to prove your ownership of the account as such to CS.