Obviously i'm partial to a gift payment given my personal fiasco a little over a year ago. However, as an "official" stance, I appreciate the site may want to state no asking for gift payments in sale threads; with a wink and a nod of recognition that members who trust each other my work out a gift payments.
Two reasons I personally advocate for gift:
1) Anyone who has filed a dispute with paypal regarding a transaction will tell you - they take ZERO account of "incomplete orders", "item not as described", etc. Their protection verifies an item arrive - that's it. Cut and dried. Meaning - you send me $2,000 as goods. I send you a box of air, with tracking. I win / you lose. Again - the extent of paypal buyer protection is they verify the seller sent you an item by verifying a tracking numbers shows an item was delivered. They do not get involved beyond that. Sometimes people confuse ebay buyer protection (which is very good, too far skewed to the buyer) with paypal buyer protection (which skews heavily to the seller). Ebay will take into account a sellers record, feedback, history of cases, history of messages sent, photo evidence and more - paypal just doesn't do that. Your goods payment is buying a false assurance. Their is a very small number of very high value sales, or sales from paypal business accounts, where it is possible to get an actual refund, but those are the exception and does not apply to 99% of deals that happen on forums.
2) The items we are selling are used 99.9% of the time. They are trading hands back and forth - they've been taxed at least one, probably more than once. Their is no need to throw good money to bad on a deal between people who trust each other. I appreciate paypals service and they should profit from the service - but they could avoid the gift payment altogether if they had a flat fee - $1 per transaction or something. This 3.5% flat fee where their work-end is the same if it's a $10 sale or a $1,000 sale is the reason people use gift to begin with.
So - repeating a tl;dr post. The site is probably wise to suggest no mention of gift payments allowed, but those who insist on goods payment privately should understand exactly what it is that their 3.5% is buying.