In my opinion it is worth to buy from a miner if:
- The seller is trusted (buy only from someone who already sold a lot of GPUs and got a positive feedback). If he sold a lot of GPUs already, he (probably) knows how to keep them in a good condition.
- Seller has restored the stock BIOS (contact him to confirm).
- There is a warranty - most of the cards should have 3 years of warranty and many miners sell the cards after 10-12months of usage (#2 is important here, in case the GPU bricks and you don't know how to or cannot flash the stock BIOS, the warranty could be avoided)
I bought 2 cards already from a miner:
- RX 580 4GB Nitro+ for 160$ in September 2018 -> but after 2 months I sold it, because it turned out 4GB is not enough for newer games
- Reference Asus GTX1080 Turbo in November 2018 for 350$ with 26months of warranty (pretty good deal at that time) and it still works without any issue. Right after it arrived, I installed a water colling (using Kraken G12) and I can recommend it.
Now you can get a GTX1080 for 300$ and GTX1070 for 220$, so if you plan to upgrade your GPU before the warranty expires, I would go for it. But if you plan to keep the card for a longer period (2 years+) it is a lottery.