Recently I've noticed tons of GPUs flooding ebay and prices seem to be dropping. The issue is that most of these cards were in mining rigs which are not properly cooled and run at full load 24/7. No seller is going to admit to this, and the reason why miners prefer consumer grade GPUs outside of cards designed for mining is because they know consumer cards will have a higher resale value. This is only the case because consumers have no way of discerning if a card has been put through excessive wear and tear. If there was say something like an odometer (no idea what would be needed to engineer something like this - or if it could be made to not easily be modified) then miners would also be more wary to buy consumer-grade GPUs as they would realize the resale value would be greatly diminished.
Miners are also likely a headache for AMD because the average miner is not a sophisticated user and are likely responsible for a large portion of their RMAs due to inadequate cooling, unsafe overclocking and the profit motive that would lead some to RMA products just in the hope of refreshing cards they know are near end of warranty. Ultimately, as defective cards end up in gamers' hands it will result in poor brand experience that is bad for AMD and will result in even more support issues. This additional cost for support will ultimately keep the prices of new cards high, but the poor performance of used cards will also create a bargain bin because of the hit-or-miss nature of cards. Why risk buying a Vega for $600 when it might only last 1 month if work at all?
Ultimately, I think the best way to prevent this from happening in the future is to give some way to have cards validated. Some way to tie a cards S/N to a test which the seller can perform online which checks how the card has been used and give it a rating that can be verified by the buyer. This could also allow AMD to deny RMAs based on improper usage of the device.