Check for bad feedback. If seller has little or no feedback be careful, check to see if seller accepts escrow service (payment when delivery is made) or try to contact personally and see if you get excellent vibes.
If you can pay through an electronic billing service such as Paypal and you can pay by credit card, this offers you more protection.
Ebay and Paypal guarantee their purchases for a modest amount (I think around $200). Paypal has a way of credentialing honest power sellers, so sometimes it's ok to buy from someone you don't know off eBay.
You should be prudent and careful, but you needn't be paranoid.

Larry