B = Van turned into an RV inside the van.
C = Cab (cockpit area) with frame sent to RV maker who then adds a box to the back that is turned into an RV.
B+ does not technically exist but refers to something in between the two. It's usually a cab and chassis (Class C) with no over-cab bed and a bigger box on the back than just the van shell. B+ is a "tweener"
So, figure out which one you want and search these. The problem is that whoever posts the ad may not know which is which and may improperly classify it in the first place. Not all sale sites have a B+ category.