Basically, you pay for the item, player-auctions processes the payment.
You then get your gold or whatever delivered to you. If you get scammed, you can file a claim with player-auctions and you get your money back or whatever you payed in credits to use on the site. However, the gold sellers are pretty legit.
I just bought 30M, was delivered within an hour. The one problem I had was some idiot was pretending to be the gold seller and trying to get me to trade the gold over so it could be 'detraced'
I think this is basically how it works, although some details may be a little off.
Hopefully this helped
1. Seller makes a thread selling gold.
2. Buyer puts in how much gp he wants.
3. Buyer pays playerauctions.
4. Seller gives gp to buyer.
5. Playerauctions gives money to seller.
If the person you are looking to purchase from is on Sythe, then you can PM them, or add their MSN. If they are not, I believe you can find the seller's contact information in his store.