I've got to say I'm a little over the older used stuff for now, especially in bodies. I also had to return a lens not long ago to a store and while the problem was reimbursed no problem, the quote "Ex" bargain I thought I was getting was not so Ex. Best to buy the best there is and figure old is old, so matter the appearance.
These days, buying anything, even new gear without actually laying your hands on it can be a real crap shoot. That's the primary reason that I buy factory refurbished whenever I can because it assures someone at the factory went back through it and fixed what they didn't fix when they built it. Plus, it still comes with a warranty and a cheaper price tag while working better than it was new.
OTOH, when I score something used, I go an outfit I can really trust that I know isn't going to try and rip me off, run away, close shop and split to a foreign country and will stand behind or at least next to, what they sell. I've dealt with KEH.com and never ever had a single issue that they didn't address promptly and that was only twice in twenty years of buying and selling through them. When they grade something, it's always under graded. I've found for example, their bargain grade is what anyone else would consider good to very good. And their very good is like new.
Dealing with a reputable outfit takes a lot of the risk out of the deal, no matter who the seller is. Waynecrider is absolutely right. You tend to get what you pay for and while I hate to say it, these days, no matter what you're buying it's still the old adage "Buyer beware" and "If it sounds too good to be true, it probably is."
Places like KEH provide solid guarantees on all their used equipment. They'll either fix it or replace it with something comparable, take care of return shipping and do it quickly. They have an in house service department. And you have to remember too, commercial resellers like KEH are in business to make a profit and have an overhead to pay for. I don't mind paying for quality a little bit extra for used gear if I think it's going to be reliable and do what I need it to for a reasonable time after I get it.
One other thing, is look for used equipment at authorized repair shops. David Odess is a Hasselblad repair guy and he sells used gear that he warranties. Midwest Photo Repair in Michigan is a Nikon Repair Center. And they sell used equipment, bodies, lenses, etc. Very excellent outfit for Nikon repairs, also warrantied fixing.
Drop the dime(s) and pay a bit extra. Bargains often break your heart, waste your time, cost you money and have you ever known a piece of equipment that didn't break at a really bad time?