Its an issue most people don't worry about because they're running exactly 1 GPU, which they wanted because they wanted to game, and if it dies, it dies.
But Google, Amazon et al. are very familiar with the concept of hardware lifetimes. Internally, calculations for some algorithms are done based on MTBF values for components - i.e. the cost of doing a thing is a certain number of CPU-hours, and you know a CPU will give you X-hours before it's liable to be hardware-dead.
A second hand gaming GPU has a very different power-on profile to a mining GPU - a gamer is going to struggle to on average put more then 4 hours a day or so through a card, most likely less. A miner is going to have that thing powered on the whole time, permanently. Even if the GPU is fine, the DDR RAM has been aging and that stuff fails faster.
The short answer is: people are going to be asking for a discount based on the expected lifetime of ex-miner hardware, once we know what that is. If the answer is "well it might last 6 months or it might last till you replace your machine" then the price will now reflect that expectation and risk profile. Not to mention the dilution factor which will be actually failing hardware being sold off by miners without properly marking it - after all, you might as well let someone gamble that bit flips in the GPU only lead to graphics aberrations rather then bad hashes.