I have always felt comfortable pre-paying for items being sold by folks that have been active here for some time, and have built up a good amount of positive feedbacks. The items were always shipped immediately upon receipt of payment, and were always exactly as described. I have also sold a number of items, some of which were quite expensive, and received payment before shipping. I think the feedback feature here does a lot for raising the comfort level for both buyers and sellers. Of course, feedback alone does not give you a rock-solid guarentee of anything. Speaking with the seller over the phone can also help give you "feel" of the type of person you are dealing with. If something feels wrong, then don't buy. A lot of the frequent traders here are avid audiophiles, and love talking about thier systems, and the gear they are selling. I have made quite a few internet friends that I still corospond with as a result of trading here.
If the seller is a new member with no feedback, then I would need to receive the item before sending payment, or the seller would need to agree to using some type of escrow service. If they don't agree, then keep looking. Most honest people would understand the buyer's need for protection.