but do MFDB have shutters like DSLRs? Does the number of activations matter?
No and probably no again.
There are no moving parts to worry about so mechanical failure is not an issue, If you do buy a used back and it does go wrong though, I'd have thought that you'll be looking at a hefty repair bill and a long lead time on the work too.
If you find something your interested in and it's not still covered by the manufacturer I would talk to them and see if some kind of purchasse cover is available, preferably including a loan back too.