I don't think you should worry about all that, should buy a used DMM from some trusted seller on ebay or some where, someone with good ratings, and then u should also see they should have a normal price, not something way cheaper than all the other good sellers.
As for the age of some DMM, if you really want, you could ask the seller to open it up and get some date codes off the IC's.....but a lot of big time seller's might not have the time, or staff to go doing that. Maybe there's even a serial code on the DMM PCB, with a date code., IDK fluke stuff, I don't own any yet, wait now, YES I DO, of course I do, just not DMM's, I have 3 Fluke Oscope's.
If I was selling a DMM, I would check date codes if I had the time, so yeah go ahead and ask, maybe they will.