They are unable to do much. Look, their systems cannot distinguish them - not since the crooks figured out the phosphor tags. They aren’t detecting any during the course of business. There are no prosecutions of “duped cobsumers” buying cheap stamps claiming to be real online as far as I can find.
It doesn't seem too weird to me: Selling someone fake stamps is a general act of fraud, between buyer and seller, and would be pursued by state/federal attorneys general.
The USPS becomes directly involved only later, when someone tries to defraud them by using a fake stamp.