About half the gear I’ve bought over the last 5 years is used. I’ve haven’t yet (knock on wood) had a bad experience. I have walked away from a few deals when my intuition told me something was fishy.
To me the first thing I look for is a sense of integrity from the seller. If I know a seller has lied to me or skirted the truth, I’m out. No matter how little the lie, without confidence in a buyer’s word, I’m gone.
An example, there was a guy selling an item some distance away for a very good price, third-hand. I asked for pictures. Pictures were great. However, I know the pictures had been modified to cover up some cosmetic blemishes. I passed. If the supplied pictures had been un-modified (showing the blemishes), I would have gladly completed the deal.
After that, it’s condition (both cosmetic and functional). Pluses are original packing materials. Modifications are red flags (unless a vintage item where a refurbishment is appropriate).