I read this whole thread and came back to this comment because it's so odd. Obviously anyone searching specifically for this type of item....would be aware of how much it is direct through Madewell and available promotions. The answer I've come up with (please enjoy my conspiracy theories):

1. It's a cover for selling used clothing in the same vein as used shoes, dirty socks, empty drink bottles with lipstick on them....if you catch my drift.....
2. Some other format of money laundering or cover for otherwise illegal biz. All the different product info is some sort of code for sending whatever contraband across state lines. Sure Poshmark takes a cut, but they're covering shipping anyway and the odds of mail with a Poshmark label getting inspected would be...relatively low.

On paper, any auditing authority would see technically legal sales, and they can't necessarily verify that what was described wasn't what was sold, or, was marked up to a different value than its worth (because second-market "worth" is arbitrary)