Make sure they have a good feedback score that is at least 100+ transactions as a seller and pay with paypal or credit card. I bought my 770/80's off ebay and had no problems. I buy a lot of stuff from ebay actually and I have only had a problem twice maybe.
Some models are more likely to be fake than others, so research your model and see if there is a thread about fakes of said model.
Look at the feedback of the seller, look at the negatives and neutral, and look at the comments.
I always prefer to buy from ebay stores, rather than just an ebay seller.
Also if the deal is too good to be true, there is most likely something fishy.