Hi,

Chain rings hardly ever need replacing, (except when they do), but those
with upgradeitis swap out perfectly usuable chainrings and cranks for new.
Some also change out the rear cassettes on relatively new bikes.

Generally I'd assume most bikers would chuck any chainrings or cassettes
replaced due to wear in the dustbin, rather than trying to mug someone.

So generally I'd assume any used cranks or cassettes are OK, and
the seller will generally give details as to why they are little used,
or in the case of some chainrings / cranks how much actual use.

Caveat emptor still applies, but IMO not really to most cases.

rgds, sreten.