Congrats on your purchase of a MFDB.
From my (limited) perspective, as I read the boards, look at dealer lists, and scan eBay occasionally, it appears that there are at least two tiers of used backs. There are pampered kits, low actuations, maybe some warranty, no dings, etc. which sell for a premium; and there are vigorously used (though functionally fine) kits that are being upgraded by pros or rotated out of rental programs, etc. This is pretty normal in other types of gear as well.
In my opinion, the high actuation counts is mostly immaterial, since there are few moving parts, other than indicating the number of times in and out of a bag, possible button condition, etc. Still, my personal preference is for new or low-use gear.
It does appear to me that one can find some great deals for somewhat heavily used backs (which should still mostly perform fine), but like autos and other things, condition, warranty, service, etc. should be factored in – and is typically a personal matter as to how much weight it carries in terms of value.