Mining is neither here nor there AFAIC. The main things to ascertain are pretty simple. See if they can tell you how long they've been running it for (can easily be proven to a reasonable level by asking for part of the original sales invoice,) maybe some details about their setup - pictures, if possible. Also look into the warranty policy for that particular card with the manufacturer in tandem with this. A warranty that you can utilize adds a lot of potential value and peace of mind. Plenty of other simple little things are helpful, too. What temperatures were they running at? Climate controlled environment? Enclosure type? Stock, or undervolt? How about fan curves? People always talk about undervolting, which any smart miner is gonna do to minimize power consumption, maximize gains, and run their rigs for longer to ultimately mine more cryptocurrency, but GPU fans are not chosen specifically for greatly prolonged operation at high RPM's like case fans or CPU fans, which are expected to hold up running 24/7 for years and years. With the GPU's they factor in the downtime - they're supposed to stopped for more than half of the time you have your card, unless maybe if it's a workstation card. They wear out much quicker when the cards they're cooling are huddled together and put under load for months on end. If anything fails, it'll be a fan. Ask any miner how many fans they've swapped. Some cards are really notorious for this. And that's important to consider, too... how easy would be it be for you to get replacement fans for that particular card? How easy are they to change? Decent chance you may have to one day. Might not hurt to inquire with them about this. If they're really good, they might just know where to get replacements.
Honestly I wouldn't worry about the other components, unless you suspect that the miner is incompetent or something. Any decent card should be able to handle that workload for a long time - mining is not intrinsically neglectful of the card's limitations and often they are run under spec. You just have to do what you can to verify that they are telling the truth. Come from enough different angles and sketchy stuff becomes more obvious. Sooner or later, something won't jive, even if it all sounds good on its own.
Any competent miner should be able to fill you in on these things and even be pretty specific. Easily and readily. Mining can be a pretty significant investment, and often it is not even worth the time unless you really monitor and configure the shit out of your rigs. It's a long haul game and most people I would hope realize that going in and plan for it, knowing that it's basically like their introverted child.
May make you feel like you're being the asshole but if they're honest and they know what they're doing, they'll be able to answer questions like this and might even be happy to share their endeavors with you. Maybe you learn something in the process. Or maybe they'll get cozy and slip up. Or maybe you even learn that they never really knew what they were doing, it didn't turn out like they wanted it to, and they dropped it the same way they picked it up: like a kid with a toy. These are things you can easily get a read on simply by getting them talking about their rig.
I dunno, at least if they can't or will not go into details about operating conditions from start to finish, that's a big red flag, to me. If you're going to take someone at their word, it's really important to examine the person behind these words and the value of the information they're actually giving you. Watch out for people dumping a whole bunch of crap on you that really is meaningless, just in order to distract you or make you think they're on the up and up. People are shitty like that - beware the smooth talkers and make sure you get solid answers to YOUR questions, not just a mountain of all of the information THEY want you to pay attention to. And remember that what they ARE directing your attention to may tell you a little about what they don't want you to ask about...
Someone who's enthusiastic about what they're doing and cares about more than just the money (or at least values the money they throw down) will show that in their general knowledge and awareness of their setups. Chances are far better they've been keeping up on their gear and treating it nicely. They won't price the absolute lowest, but that's not what you want anyway. Someone who sells cheap doesn't care and never did. All you will be to someone like that is an opportunity. Whereas the guys above them would probably rather sell to someone who kinda knows what they're getting and appreciates it for what it is.
...basically, just be smart about it. Be engaged in the interaction, but take steps back, too. Just showing interest can buy you time to investigate on your own. But really I suppose this all goes for most things in life were goods or services are on the line. It pays to always remember though. Even the smartest people can get burned by a deal that's too good to be true. Think of it as covering your bases. The more questions you ask now, the more ammo you have later, if things go sour and it turns out things were misrepresented. Better still if you have these interactions documented.
There are equally as many ways to neglect a card in a regular PC, and either way you will be taking the person at their word. Which is like... hey, at least they've told you they were using it for mining in the miner's case.
With GPU's in general I think it's best to stick to trusted sources. That'll always be your best bet, mining or not. Mining GPU's can be a great way to save money in this stiflingly lame GPU market, but like buying ANY used piece of delicate equipment, it also pays to know the ins and outs of how they may be used, and things people might do that have damaged them without them knowing. That's more than I can cover in one post, lol. I'll just say, the more you know going in, the more you'll be able to make good judgement calls on.
Personally I bought a mining RX580 from our very own cdawall a while back and it has served me well. And for ~$150, if it lasts a couple of years I'll consider it my money's worth. I guess that's really the other thing. What is the used mining card to you? If this the best you're going to be able to get for a long time? Or are you just waiting for bluer skies? Latter makes mining cards a lot more appealing. If you're really counting on this thing working, or at least getting your warranty, I'd be very picky or better yet save for as long as it takes to buy new. Basic buyer protection goes a long way. Any way you can to have things traceable is always best.
Local seller has its benefits. If you're lucky you may actually get to see where they ran the card, not to mention you can get a real good look at the card itself before you buy it.
