When I visited the Phase One factory last summer I specifically asked their chief engineer and head of manufacturing about this issue.
Their response was a definitive statement that sensors and related electronics suffer essentially no wear and tear, and that even hundreds of thousands of exposures will show no effect.
One thing to remember about the Phase One backs is that they use a "sleeping" technology. What this means that is each circuit is powered down to its lowest possible state except for the moment that it's needed. This means that these backs run very cool, and as we know, heat is the enemy of electronics.
As long as the back hasn't been subject to physical abuse I'd have no problem with buying a used one.