Why would you need to resell them when you get Nitrox? You don't need brand new tanks for Nx. At most, you may need a tumble and O2 cleaning. If you are buying new, it's probably OK to go from there. The VIP sticker should reference the cleaning status. Generally you'll see 2 or 3 options. Air Only, Nitrox Clean, or O2 Clean. I'm not sure if Nitrox Clean is actually legit, though. It's intended to be used only for banked Nx. So maybe they swap out the O-rings for Viton, but that's probably it.
O2 clean is needed for tanks that may be filled by partial pressure. My tanks are all O2 cleaned. Most often, they are filled from banked Nx, but sometimes I'm away from home and the shop may use partial pressure.
I can still fill with air, but it should be O2 compatible air. If the shop does Nx fills, their compressor air is probably fine.