I wanna get a full frame. Thinking about 5d mark 3 or Nikon d610

Not sure which tho, as I don't know which have the cheaper used lenses, as they both look pretty equal to me I just care which mount has the cheaper ones when buying second hand
Buying most shit off eBay so use that as reference
(Would consider an a7 but I'm not paying 700 for each lens)