Remember, if the price appears too good to be true, it probably is.
Vet the seller; what's their feedback like (as a seller, not as a buyer) ?
What are similar spec'd / condition MacBook's selling for?
Read the ad. Is it defective? Spares/repairs?
Is it an auction? Typically, MacBook auctions start really low (even the ones spares/repairs) and will jump up substantially over time.
I would say if in doubt, avoid it. But eBay/PayPal's protection is really good - so if you can afford to risk being out the money for a while, it should be returned to you if you file a dispute.