I'll start with the positives, poshmark is a huge plate form for country wide users. They have a large amount of traffic so the chances of selling your stuff it likely!
However, their rates are extremely high! They take 20% pre sale plus for some unknown reason you have to pay taxes on the poshmark fee. I'm selling used items/clothes and as the seller paying poshmark taxes...? This is not a business, if it were I'd have my own "business" website to sell. I've tried looking online for a reasonable explanation for these tax fees and there isn't.