I haven't had any issues with used measuring devices, but I am very particular with the ads and sellers. If anything suggests that it may not work, or the seller seems not to know much about the function I pass. The ones I have are marked from commercial use, and well used, but fully functional. It is quite possible that they no longer meet specs for certification but they are more accurate than my means for testing (mostly measuring parallels of a known size). They may or may not be more accurate than my cheap tools, but they are clearly higher quality and nicer to use.
These are expensive tools for us as hobbyists but for a working shop they are semi-consumable items. My guess is many are taken in trade and then cleaned up or even refurbished by the seller. Most of mine have come from ebay sellers who had a variety of used machining tools available. Maybe not as cheap as a seller with a micrometer mixed in with Beanie babies and baseball cards, but I think their description is probably more reliable.
I posted a video in another thread that was somebody going through a counterfeit Mitutoyo and an comparing it to an authentic one. The level of detail to distinguish the fake from the real was pretty amazing. They were looking at things like the included battery being the wrong type / brand, things like the serial not matching the certificate (which is just a copy of a real one). The big give away for many are when their Japanese made tool ships from China...
