Since I plan on purchasing a new daily driver, this is a good thread for me to soak up all the information. I'm thinking I'll go the route of using my ATM card and pulling several maximum amounts until the purchase is complete. That way, I show up with no cash, can confirm the car is what I expected it to be (not falsely advertised or non-existent), and it only costs around $10 more to receive money at any ATM machine. (several transactions at ~$2.00 each.)
As for selling a car, I'd say the best bet is to expect money up front in whatever payment plan the purchaser selects and waiting until a check clears or you have money in your hand at the time of the complete transaction.