I have bought from multiple sellers, and I have only had one case of a bad-faith seller in that they were out to scam the buyer. Most sellers are selling clothes to make a small profit since Poshmark has a considerable commission off each sale. I advise someone who is a new posher to have a conversation with the seller under the public listing if you are unsure about the purchase. Make the seller state if there are any stains, defects, or sizing abnormalities.
The bad seller I did have an issue with listed a child-sized skirt under a woman's sizing. Poshmark sided with them after five days of review, so there is no safety net for buyers duped into buying children's clothes the seller listed under adult sizing. This is absurd as I uploaded a screenshot of the brand's size chart, but it was filed under a "fit" issue. No adult woman is going to fit into a small child's skirt.