When I buy from private sellers over the net:
For amounts < $1K, I just send over the UPSP MO. However, before sending I make contact w/ the seller and talk to him. Also, check his name/address with reverse phone lookup at:
http://www.reversephonedirectory.com/
If anything doesn't feel right, I'd terminate the transaction or request that he go thru a store front FFL to complete the deal.
Anything more than $1K, I ask that it be done thru a store near the seller and I picked out. I find the store and call them and ask them to help check and ship the item. Again, if anything doesn't feel right, I'd terminate the transaction.
When I sell an item, I also give my home phone # so that the buyer can do a reverse lookup on me, and I always ask for MO payment.
If it is FTF, then its always done in cash.