It has happened to me where after the buyer confirmed full delivery he told playerauctions that it wasn't actually delivered. I had to upload screenshots and it was eventually resolved in my favor.
When using PlayerAuctions always take pictures of the trade, you'll never know when you might need them. Although 99.9% of the time I don't tend to have any problems with PlayerAuctions.
Vouches are more of a sign/proof that you've traded with this person rather then they're trust-worthy. I mean just think of all the people that are banned which you may deem trust worthy, vouches are a sign of an experienced trader IMO.