1. Ensure the poster has a few posts. The more posts a poster has, the less likely this is a driveby scam.
2. Ensure the poster posts a picture of the product, complete with a handwritten username, date and preferably "Tom's Hardware written on a piece of paper near it.
3. Check the shipping costs
4. Consider an escrow service or at least a traceable money transfer method ie. not wiring money
5. If the deal is too good to be true, it probably is.