At least for camera equipment, this does exist. You can take your camera to B&H's used department, and they will buy it from you. You can also go to B&H's used department and buy a used camera from them.
The advantage of this over Amazon is that as a seller, you know B&H isn't going to screw you over in any way, and if they do, you know where to serve your lawsuit. A trusted business with a physical location is the counterparty in your transaction, which makes it pretty low risk. As a buyer, you know B&H is trustworthy and you can come back to the store and have a problem resolved.
Amazon offers nothing in a typical Amazon.com transaction beyond matching buyer and seller. They can't trust the buyer, and they can't trust the seller. So they can't really help with the transaction in any way, they are just there to take a cut for basically nothing. B&H takes a larger cut, but they actually do something.
Of course, people like money, so Amazon is rolling in it while most people don't even know that they can sell their camera to B&H. (I've bought used stuff from them because I don't trust random eBay listings, but never sold anything because the amount of money they offer wasn't worth it to me.)