As far as I can see Poshmark is a site for selling apparel so when you say you are a "reseller" you are buying apparel from others and then attempting to resell those items, at a higher price, on Poshmark?
If that's correct, then the only "taxes"I can think of that Poshmark could possibly be taking out of sales are sales taxes, and sales taxes really aren't a topic of this forum. There's no way that Poshmark could be engaged in income tax "withholding" since your relationship to them is an "independent contractor/sole proprietor", a situation where there's no income tax withholding involved.
From an income tax perspective you certainly can deduct all "ordinary and necessary" expenses you incur in trying to make a profit from your activity.
