There really isn't any customer service. Sellers are protected. The worse thing that happens to sellers is they get the item back. You could leave a one star rating and an explanation of the problem but no one sees it but the seller. If they are trying to become Ambassadors they might care but there are so many sellers that aren't that serious ruining it for the good people. There are some great sellers who go out of their way to please. Then there are the ones that take advantage and just take your money and fight you if you want to return something. And it is your word against theirs so you better have good proof. There is a five star love note system and that is supposed to show people who really pleases but I have had sales from non Poshmark people who never return and didn't leave a rating even though I have only five star sales, so that doesn't really show much. In the end there is no rating system so people don't have to have integrity to get by on Poshmark.

I'm going to be quite honest about PoshMark since I have been on there for 3 years.
I've had a lot of luck with Poshmark. I've only been scammed once but that hasn't stopped me from using poshmark. I no longer trade due to being scammed. I follow all poshmark rule's. I've bought and sold on poshmark. I once bought a watch, it looked like a really good watch. When I got it, I took it out of the packaging and it broke when I put it on. I contacted poshmark and I was able to return the watch and get my money back. Never dealt with that seller again. It was just a process waiting for answers because PoshMark decides if the product in question was described as you got it. They ask for pictures of proof. My other experience was I traded with a girl. I tracked my package, I know she got it because it was delivered. She sent me a receipt from the PO saying she shipped out my package (We shipped the same day). She cut off half the tracking number, when I asked her to take a picture of the whole tracking number she claimed she had already threw away her receipt. She never shipped out the package. I was completely out of an item I could have sold. Advice, follow posh rules. Do not trade. Read the "love notes received" section on the seller you wish to purchase from. Happy customers? More than likely you'll have a good experience. You can't fake "love notes" on posh.
Read ALL of the posh rules and FOLLOW them. It is VERY important. Just follow the rules. It helps to not get scammed. I got scammed because I didn't follow the rules.
I also had ONE other problem with them. A buyer didn't pick up the package from the PO because the box was too big and the PO told her she would have to pay extra. I contacted my PO and had the package sent back to me. (PoshMark keeps a record of this, the product still says SOLD in my sold section yet I have it for sale. There's no way you can delete it. But it hasn't affected my selling like I thought it would.) I got my product back, and the buyer got her money back. She didn't repurchase the item.
Try and use small boxes without squishing items.
Two lessons learned for me.
Package items with care. So they are not broken in shipment (because then you'll get broken items back.) Poshmark protects buyers. If don't describe the item exactly, the buyer can return it through PoshMark. You have to be 100% honest with your description to ensure happy buyers.
Don't request direct deposit for your earnings. Get a check. (I personally, do not trust direct deposit through posh. I believe checks are safer. Even though they can take up to 3 weeks to receive them.)
Scammers are out there.
Everyone will get scammed at one point on poshmark. But I do think that the benefits out weigh the risks.
Good luck.