I've used PayPal for years both as seller and buyer. You get a legally valid receipt with each purchase, just as you do with credit card payments. So that part is not anything to worry about. You can just print it if you need a hard copy for tax purposes.

The use of a model or any other intellectual property in a game will fall under the EULA or any other user agreement you accept by purchasing and downloading the model etc. It's standard procedure. You can of course also set up a specific usage contract if you want with the owner of the model etc.

cheers,
tindust