Me and Lups moderate the marketplace daily. Threads that don't meet the requirements are always closed. Truthfully, 2 months and 100 posts isn't going to discourage anyone who wants to scam here. We've had longstanding members decide to scam one day and leave the site. The most we can do is make sure the site's requirements are met and that people follow the rules laid out in the sticky's. No amount of time spent on the site is going to dissuade someone from scamming another member if that's what they've set out to do.
There is already a warning at the top of each marketplace section about using the gift option on PayPal. We clearly explain that you lose all buyer protection by sending your money as a gift. If, after knowing that information, a buyer is still comfortable sending payment as a gift, then I have to assume they understood the risks involved and were comfortable doing it anyway.
Most sellers seem to say that they are fine with a gift payment or a standard payment plus 4%. I think that's fair, and if someone does the standard payment plus 4% then it covers the transaction fee and the seller gets what he wanted for the item.