I have seen only quite a few threads recently about listings being suspicious but im certain there have been more that i just dont see and it has been a problem that players just have no idea how to report them or that there even is a way to report them at all.
Go to the listing ID on the left side of the flea market UI, right or left click the ID and hit Suspicious Listing
For a bit of clarity:
This thread was not created to get players to scour the flea market for suspicious listings and report them.
It was simply to inform the uninformed players about the feature, which over the course of the last two years felt like a large majority.
Please don't just go hunting for "suspicious" listings and just report the ones you find more organically. It really removes the potential validity of those reports being acted upon.