It depends on the dollar amount, in my opinion. I'm not going to bother getting my attorney involved over a couple of thousand dollars or less. I'll whip up a general contract and have the buyer/seller sign and mail it back to me and then either Paypal or wire transfer the money to me. Its not a very solid way of ensuring you won't get ripped off...but it boils down to the amount of effort that I'm willing to put in to protect that small amount of money.
Anything over a couple thousand dollars and under 10k, I'd talk to the person on the phone and research them - as well as have my attorney draw up a formal contract. I'd then do the mailing bit with the contracts and have them either pay via escrow or wire transfer. If I felt that anything didn't feel right, I'd pass on the deal.
Anything over 10k, I'd meet face to face at least once before the contract signing and I'd sign the contract and get payment via certified check or wire transfer promisary note...in person. I'd also consider having both parties attornies present.