is it worth it buying it?
No.
Here's GamersNexus answer to high GPU utilization (3D rendering or mining) and what can blow up (at 06:21):
View: https://www.youtube.com/watch?v=3WWfj7RF_z8#t=6m21s
And GamersNexus answer to if to buy mining GPUs (at 11:42):
View: https://www.youtube.com/watch?v=g1Yp6pQRdns#t=11m42s
Way too expensive for a mining GPU.
Brand new, RTX 3090, in a sealed package +
warranty has MSRP of $1.499.
Now, is 750 bucks good for used mining RTX 3090? Not by a mile.
To me, any used GPU, which i could buy, has instant 50% off of MSRP since the GPU doesn't come with a warranty and i have to take a risk. So, for RTX 3090 at MSRP of 1.499, 750 bucks is what i'd pay for.
And i even don't look at mining GPUs, unless their price is good
for me. Meaning another 50% off of used GPU price. So, for used RTX 3090 of 750 bucks,
375 bucks would be what i'd pay for mining RTX 3090.
Lets say you fork out 750 bucks for that mining RTX 3090. You can use it for e.g 2 weeks (or 4 weeks, or a month) and GPU dies. Then what? You do not have any warranty, so, no chance to get GPU replaced. And i doubt that GPU seller is going to refund you either (since they have 0 obligation to do so). At this point (if not far earlier), the reality will hit, that you should've gone with new GPU instead, for the redundancy it comes with (warranty, usually 2 years). Unless you have money to spare to get yourself 2nd working GPU.
Lets look at it on another perspective:
Why miner is selling the GPU in the first place? GPU is still a good "tool" to make money. Unless, there is an issue with GPU and it isn't "good enough" for the miner anymore. Perhaps miner also knows the signs of GPU starting to "fade out" (die), and it's time to get rid of the GPU. Preferably at the same price as MSRP, (without incurring any loss, but keeping all the profits GPU made for them).
All-in-all, regarding PC hardware, never buy used: PSU and GPU. Unless you have the money to buy 2nd one, once the used one dies on you. Though, on PSU part, things are worse since when PSU goes sky high, it has the magical ability to fry everything it is connected to. Aka your whole PC. Hence why
never buy used PSU, since you don't know what kind of abuse it has seen.
Another thought;
Miners are the sole reason why we have GPU prices so high and why almost all RTX 3000 series GPUs were sold out at launch, after which, scalpers put some of them at sale, 2-3 times of MSRP. Helping miner to get rid of the useless stuff (read: burned out GPU), isn't doing any of us (gamers) any favors. Instead, it shows miners that they can get away with it and same happened again, when RTX 4000 series launched.
So, besides risk to your wallet, there's moral reasons as well. At least to me.