Have you sent an email to Poshmark support? I have before and they did respond

You should email them. Did You purchase anything for Yourself after You sold the items? If so, the amount You made ($30) would be used towards what You bought. I always request for a check. Takes longer but I forget about the profit & buy things thereafter & waa laa... I had My earnings still there.

I don't think there should be a problem with trying to cash out. Have you checked to make sure your buyers did not initiate any returns and your funds retracted to issue the refund back to the buyer? If not, I will suggest you send them an email, they usually respond within 48 hours. It's better than losing money.

There is no phone service from postmark. Consult their online customer service and expect to wait a several days.
They are a waste of time, and in your case money