Poshmark has a really BAD reputation when counterfeit items are involved. They almost NEVER act on rteports of fake items, or they let one of their "authenticators" who may be anyone from a professional authentication service to a PM seller with absolutely NO authenticating experience. And their professional services such as Entrupy aren't any better, we Coach authenticators here estimate that they're correct maybe 60 percent of the time. That's a bloody poor record for a paid service!
These forums are full of posts about members' problems with Poshmark, not just the Coach forums either. One of our Coach authenticators won't even authenticate their listings because she refuses to be even a distant party to their total disregard not just for US and international laws regarding the selling of counterfeit items, but also because when some of our members have bought fakes, Poshmark often refused their requests for refunds even when solid proof was included. PM's "authenticators" have also been known to identify fake bags as genuine - Entrupy is especailly famous for this.
In short, you won't find many Poshmark fans among tPF's authenticators, regardless of brand. JMO.
There's also a 262-page thread in tPFs Ebay Forum just on the subject of Poshmark and its users' opinions:
https://forum.purseblog.com/threads/does-anyone-use-poshmark.780202/page-262