Depends on the price and who you are buying from.
Know the true price. Look at completed auctions; in green you will see what such a card has sold for.
Use normal precautions.
Buy from a seller with good feedback.
Pay with paypal. Ebay sides with buyers in disputes.
Look for a EVGA GTX1070 that has been registered and is still under a transferrable warranty.
I often sell my old parts on ebay.