Having said that, the culture on Poshmark it just weird to me. If you read the comments, most times people are asking to trade or to go off-site to complete a purchase to get a better deal. Poshmark doesn't seem to care about this, which I think is odd.
Supposedly, and it's in their terms, they will ban members who they find coordinating purchases off posh via mercari, paypal, instagram, etc. This is why when people do it you'll see them using emojis or abbreviations to spell out words like mercari (with the M emoji)/merc, PP for PayPal, etc. I see it happen often though and a lot of these people are still selling.

They don't mind if people trade but they don't encourage it since there's no way to guarantee you'll receive your item.
Personally I'm starting to steer away from Posh. I sold some Anthropologie pants on there a few weeks ago in pristine condition, the seller received my item and filed a claim against me saying the pants were dirty and stained, which was not true. The pictures she posted did not match the pictures in my listing, which didn't matter in the end because Posh does not protect their sellers. When I received the pants back, they smelt like they had been worn and had new stains on the crotch (ew) and on the leg. Clearly whoever bought my pants wore them, decided they did not like them and since you cannot do returns on Posh for this reason, decided to file a bogus claim against me. I filed a claim myself and Posh luckily paid me for my sale. But it was still an extremely frustrating experience.