If you're seriously looking to scan a decent amount of 4x5, It'd be worth it to just spend the money on a new v850. I know people don't want to spend the money, but at the end of the day you have to ask yourself how much your time is worth. You could probably already have one and be happily scanning along instead of burning time trying to find a deal. Buying used is fraught with all kinds of problems as you've been discovering and more often than not, results in lost money and lost time. I would not consider buying a used scanner unless it came with everything a new one does in the box, AND had a return that allowed you to return it if it didn't work. Anything less than that and you're risking money and time.
It's a lot like people who drive around parking lots trying to find the closest spot so they don't have to walk as far instead of grabbing the first spot they see and walking in, doing their shopping and walking out. I fairly often just grab the first spot I see, go in, grab the thing or two I need, then check out and walk back out only to see a car still circling the lot (that was circling the lot when I got there) waiting for a close spot to open up.
It comes down to how much your time is worth.