The last camera I bought brand new was the Nikon D7000. It went berserk after I soaked it in rain during a cyclocross. I sent it to Melville to fix it but then it wasn't a good fix. I remember sending it back again as the AF wouldn't light up or something something. It was many years ago. Since then I've always bought used.
From eBay to retail I have almost 100% positive experience. The items are good and if the items aren't good or good enough to live up to my expectation the sellers are willing to take it back no problem at all.
I shopped at
KEH, I bought a Nikkor 28mm f/2 manual focus lens, the glass elements have lots of bubbles which isn't exactly faulty for good old lenses but ... they were willing to take it back. I am sure I bought some filters and other stuff no problem like some bazooka.
Adorama. I bought some big ticket items like my D4s with them. No problem. I bought a Good lens with them. Their Good is not what I would call it Good in layman's term, check out their definition it could have problems. But you want to think they are being conservative. Anyway, I ordered Good but it has 3 flaws. The major one was there was a scratch on the front element right in the middle. The lens where it meets the lens hood was out of shape, the lens hood couldn't screw in properly at all. So you don't know what the lens went through. Did somebody dropped it or some idiot screwed the hood the wrong way by brute force. The tripod foot was so tight I didn't want and couldn't unscrew it. The scratch did show up in one of my pictures out of some 1,600 frames. Adorama lists items with generic pictures of the items so you can't get a feel what it is like plus it won't go to describe individual item unless it's big I guess as even Good is defined as bad in layman's term. So you can't really say oh it's supposed to be Good. No. Not as defined by them. But they have good customer service, return it no problem. But I wish they would have mentioned the flaws.
Usedphotopro or Roberts Camera. I shopped directly or via their eBay store. Good experiences. Good items. I did try to return the item but they say don't even bother, just keep it and issued me a refund as the item is not functional. Probably my fault as it says untested. They didn't say it's non functional.... I bought some Excellent item and it does come in excellent. And the price is very competitive.
Buying used or selling used is about managing expectations. When you say Good, the average person would expect good as in good. I don't mind the body is roughed up with brassing and all. But for the lens has a scratch right in the middle I think it needs to be disclosed. That's just my opinion. Just remember they are used so don't expect pristine through and through. Sometimes you got lucky sometimes the Good is just as defined by the vendor.
Overall if you aren't made of money, buying used is not a bad choice and sometimes it's the only choice.