What Mike said...
A Nitrox (EANxx) tank is merely a normal air tank that has been cleaned really really well....and it thus certified for partial filling with 100% oxygen (which provides for really quick oxidation and fires, if not treated with kid gloves...). Air filling stations that do Enriched Air (nirtox) may fill nitrox tanks differently....Some provide "E" rated air/EAN that is hyperfiltered dry.
If you put air in a nitrox tank, you peel off the label and you are good to go for using the tank with air only.
However, as Mike said, I'd recommend the Visual inspection, but I'd do it as a condition of sale...That is, if the visual doesn't pass for some reason, fixing any problems is the responsibility of the seller.
If the reg can be used with Nitrox, it can also be used interchangeably with air. However, there are probably air regs that cannot be used with Nitrox. It is my understanding that the O-rings are different in regs that are usable for Nitrox/air, as compared to those that are just usable with air.
I'm sure there are experts here that will point out things I missed....