1) Get as many photos as you can from the seller (especially ones of blemishes).
2) Speak with the seller on the phone.
3) Tell the seller how to measure the frame, and get accurate measurements of all tube lengths.
4) Ask the seller to read all labels, markings and serial numbers.
5) Use PayPal, if possible.
6) Tell the seller how to pack the bike, or pay for an LBS to pack it (contact the shop directly and ask them to ship it).
7) Insure the shipment and get a tracking number.
8) Never pay more than you are willing to lose if the deal goes south.