I was looking to upgrade to a new gpu and I found an rx 580 8gb. The seller said that it was his 7th gpu in the mining rig. Is it safe to buy a gpu that's been used in mining? Also what is the pros and con's of buying a minded gpu?
Lifespan of gpu depends on many things like temperature, voltage and usage. You should ask the seller for details mainly reason for sale, receipt and warranty. If its a trusted site go for it.
You may also need to check the condition of the GPU fan since those run 24/7 on mining rigs; and thus wear out more quickly. If the fan has quit, then there's the possibility of heat damage on the card itself. This doesn't include physical or bios alterations done to the card (as others have mentioned).