I've done this a few times for bikes I really wanted. Important to establish a rapport with the seller, and make it easy for them to want to ship you the bike. There's a lot of risk on your end as the buyer, but if you really want the bike you assume that risk. I basically offer to cover shipping, give them instructions on packing and shipping (either suggest a LBS or bikeflight), and pay up front with some kind of electronic payment method, and maintain consistent communication until the bike arrives at my place.