If you truly fear catching cancer from a camera used by a smoker, I suggest you refrain from purchasing said camera. However, I doubt seriously you would be able to even tell that fact.Or would the smell of tobacco not get on the camera because it's made of metal body?
What if they were smoking pot?
I've bought used gear for almost as long. I have an old Nikon FE bought 20+ years ago that still smells like an ash tray on a hot day. Same goes for an FM2n that still exudes a whiff of stale scent. My polycarbonate-body Nikons, a couple bought from heavy smokers, are odor-free. Some people just never actually clean or wipe down gear--something you've doubtless encountered dealing old equipment. Agree, though, it's gross.I've been dealing in vintage cameras for nearly 30 years and I can attest that cameras, lenses and other photo gear can most definitely acquire a cigarette smoke odor and wiping or scrubbing the gear only partially handles the problem. Perfumes can also present a similar issue.
I have a Z9, which is inherently a much more capable wildlife / birding camera than the D500 and my keeper rate with the Z9 is higher than the D500. However, the D500 with DX sensor size has that 1.5x crop factor over the Z9. I can't see any difference in IQ between the two using my 200-500mm f/5.6 or 600mm f/4G sometimes with 1.4x tele. I know the Z9 shoots at 20fps raw, has IBIS and eye tracking, can even shoot at 60fps in DX jpg mode, 120 fps at 11mp jpg and has an amazing pre-release feature, but there is just something magical about hearing and feeling the D500 mirror flap at 10fps as I get off a sequence of shots. BTW, the Z9's animal eye tracking isn't always reliable, especially with song birds in trees or birds with less contrast around the eye like Bitterns, so you end up using the AF system just like you would the D500 or D850 AF system. The Z9's eye tracking works best when the subject fills a significant portion of the frame and the background is not busy.