Remember to use your head and vet the buyer/seller as much as possible. Keep records, and if something seems fishy - back away.
We've had a report or two lately about possible scammers, so just use your head.
If a deal seems to good to be true - it probably is. If they have you jumping though weird hoops - that should be a massive red flag.
And if you see something fishy - report it to an admin or mod.