I think that maybe sending paypal invoices along with the trade submissions is a good idea, you can never have too much protection. I mean at the end of the day a trade submisson is only someones word. I have not had a bad experience yet but im sure there is someone who has!! Any thoughts?
Especially for bank deposits and transfers, I don't know of any way you could include receipt of payment for those easily other than taking a screenshot and uploading it everytime. I think Adam hit the nail on the head.