I haven't had any major issues buying nor selling. Had a few shady sellers that didn't properly disclose damage or poor condition, and that always worked out in my favor (other than some hassle and time).
My worst experience as a seller was actually when I sold a NEW item, as in--NEW in sealed box never opened.
The buyer comes back and says the item was new and sealed, but also defective out of the box. Since the item still had the vendor seal, I had never opened it for inspection, so I was left holding the bag on that one! Luckily, it was only ~$50 item, and once I got it back I was able to repair it and sell it as new but repaired.
Regardless, I learned a hard lesson on that transaction, and I got lucky that it was a relatively small $ amount involved.
So, today, even if I have an item New in sealed box, I must open and inspect the item. Then for the listing, I will say that the item is new and only opened for inspection and pictures.