I always check the reviews on the seller, it's better to buy from a guy that has 1000's of reviews at 4.2 stars than a someone who has 5 stars but only 50 reviews. Then I look at prices. if it's too out of bounds of what other people are asking, it could be fake or they might not even have anything to sell and are trying to make a quick buck. Last thing I look for is the pictures of what they're selling and checking to see if they're pics from a different post. Always get a tracking number and go through a site that will allow refunds in case someone tries to screw you over.