Although the idea is good and many people would benefit from an online "mining-specifc" marketplace, execution as always would be a problem.
Even if you use smart trustless contracts, there would still be a human factor involved that could invalidate the entire platform. What if the buyer says it's broken and asks for a refund when in reality it works fine? Basic questions like these are a problem not just in mining or crypto, but in majority of online orders.
There are some ways...for example seller can film his rig and serial numbers before sending it and if buyer says that rig is broken in order to get money back he has to send that rig back to seller, seller has to inspect that all serials are ok if not he refuses to receive delivery and sends it back (meanwhile he sends screenshots). In this case third party inspects their conflict and there is procedure when people decide who is right or wrong after that ETH goes to person that is right. I guess I have to post entire whitepaper to explain all possible situations...