There'd be no way to tell if it's been used for mining without the person who owns it saying so. There's no digital footprint on the video card that shows how hard it was worked over time or anything like that. Not something anyone but the manufacturer would have access to anyway. It's just like buying anything else used, you're not sure what you're getting until you use it. It would be like asking a person selling a car to you if they've ever drag raced it...You either get an honest answer or you don't, and it's up to you to judge whether you believe it or not.