Yeah, this is all familiar. Good used MB's are rare, as many people stretch like crazy to buy one, and then don't spend a bloody dime on repairs or service.
That's why I have a "waiting list" anytime I sell one of my MB's. Word gets out about guys like me that are keep their cars in true top shape.
When I buy used, I usually buy from an MB dealer. They're still sharks, but they don't keep junk on their lots. Otherwise, it's just about always from someone I already know.
I've bought one MB from a stranger in a private sale, and it was easily the worst MB I owned, as it had led a pretty hard life. That said, the car was a bargain, I racked up a whack of miles in just a few months and sold the car for more than I paid for it.
Start working the "network" to find a good used Benz private sale, or resign yourself to the dealer's lots... (all IMHO)
1998 C230 "Black Betty"