I have not made a purchase on this forum, but I have made numerous purchases privately on another cycling forum. Here's some suggestions:
-You can make a payment to someone via paypal, if you both have a paypal account. You just sign in and find the "make a payment" option. Paypal will take a 3-5% charge depending on your membership levels.
-Just as I only buy from established members on ebay, I tend to only buy from members of a board that have made numerous posts, have a positive reputation, on the board. This could be tough here, as this board is fairly new and folks don't seem to make tons of posts.
-When I don't use paypal to make a payment, I send a USPS money order from the post office. They cost a dollar, but are only available up to a certain amount ($500 I think). Once I had to buy several money orders for an expensive bike. If they are lost, you can report it to USPS and if no one cashes it within a certain time frame, they refund you the money. This happened to me once, so I know it works.
-If they live far away, you can try to find a local bike shop that will act as a broker. I use superpages.com to look up bike shops in cities around the US. The person drops the bike off at the shop, calls you to say its there, boxes and ships it, you pay them.
-For any big purchase (bike), I always request a phone # and give the seller a call. I find it much easier to get a "feel" for a seller over the phone than just internet messages.
-There are escrow companies (and some bike shops will act as an escrow company) where you send them payment and the seller drops off the item and the escrow company only completes the transaction after both parties have completed their part.