Over time, I have bought a few used cameras and gear. I have paid high prices for some used gear when the item was in good shape. But usually I would buy "user" grade gear at camera shows in order to "get a feel" for whether a particular "type" of camera would be a good fit for me. Doesn't make sense to me to spend full price for a top-quality camera, if I am just experimenting to see if it will work for me.
That's how I ended up with Linda Zackelfoose's Rollei 35 with stripped internal rail for $60 at a camera show. The seller didn't tell me the rails were stripped, and I kind of lost out on that deal because with stripped rails, a Rollei 35 might have been worth $40.
I have learned from lessons like this to be wary at camera shows. Most of the gear I have encountered at low prices, tended to have internal flaws that drove the original owner crazy so he/she wanted to get rid of it.
Another thing that came of this is I learned a bit about camera repair. If the price is low enough, the risk to digging in and trying to fix it, just isn't that great. Finding the internal workings have "already been visited by an amateur" doesn't concern me much if the transaction was quick and I didn't ask many questions (if the price was low).
But I also have bought used cameras at other venues, thrift stores, garage sales. Here, I have often been delighted by my findings. Often the camera comes with a bit of dingy dust, but "rag and towel" is all I do. Often the internals have never been touched! These are most enjoyable to discover because if I am careful, I can restore the item to almost good as new.