If the price seems too good to be true, it is.
If you have to email the seller in order to join his "pre-approved bidder list", ask yourself why the seller isn't taking bids from the general public.
If the seller will only accept money orders and wire transfers, ask yourself why he won't take Paypal or credit cards.
If the seller's feedback is full of positive responses for low-priced items, ask yourself why none of the feedback is from people who bought Powerbooks.
If the seller indicates that he's located in your town, ask yourself why he won't let you see the item in person before you bid.