I think a 3rd party is really needed, and problems which could occur, will allways occur.
You listed all the problems a 2 party private contract could involve.
Payment: yes I want to use paypal (and this is the reason why I couldn´t go on kickstarter as I dont have credit card). But we all know that even paypal invoices/payments can be taken back if you report them and this happens alot.
A pays B, B gives A the product, then A goes on paypal says that he never get the product, gets his money back, and B loses his money and product. That´s not a save method.
Shipment: The product would be shipped to B then to A. Which would add shipping time, shipping cost, doubles the risk of packet loss, gives B a chance to do some shady stuff "I have send it a weak ago, dont know why it´s not arrived" or you get a box of nothing. Or he just doesn´t do it because he is lazy, changed his mind or whatever.
Backin ID/Acc: Also risky because he could write to KS, saying his account was hacked and retrieving his account, changing everything back to where he needs it.
IMO a trustworthy 3rd party needs to be involved, as the risk created by no regulations to high. Also it should ship directly from a Pimax distrubuter directly. Kickstarter won´t work as they don´t accept paypal, and if you pay the other person with approvement of KS there still a chance to lose the money.