This article mentions a bunch of different practices, some Ok, others not, but seems to disapprove of all of them.

Listing food items you got from the trash? Definitely not ok, whether listed as new or not.

Reselling unopened clearance items you bought at Target or WalMart? That seems perfectly fine, what's the problem?

Reselling unopened liquidation items you bought in a random grab bag at auction from a liquidator who bought them as returns from amazon? That seems fine as well. Should they be listed as new, like new, or used? That's reasonable to discuss.

Reselling thrift store bought items as used? Sure why not. Tons of thrift stores list items especially books on amazon. It's one of my main sources of out of print books. Can't think of a thing wrong with this.