Mercari complaint: Scammed and lost money by Mercari.

I started selling just a few items during the covid ..2021..In 2022 I have sold a total of $214 dollars..I didn’t make $214 dollars obviously I had to purchase some of the items..Just yesterday .I posted 3 different items..I was made an offer right away for $280 dollars. When I went to accept the offer..I received a notice saying I must file a w9 before I cam sell another item..They are saying $600 dollars..I am totally baffled..When I went to message my customer that I had tried 5 to 10times to contact them regarding this issue..I have never received a response..They would ot allow me to speak or give my phone number or email to this customer. I would of like to sell him my item without mercari..It seems to me that is very discriminatory..How can you tell anyone when you are selling to customers that you can have no contact ever.Only mercari who us never available can do all the transactions..I know they are scamming people also.The very last transaction I sold months ago..I went to my Usps main branch sent the item out..In 3 days the item was received by my customer..I was waiting for feedback or the 72hrs..When I finally got paid they took additional funds of my sale by saying. I didn’t have enough postage on the item..That is absolutely illegal..My aunt works for the postal service..If their wasn’t enough postage..The item should have either been sent back or a pink slip would go to my customer letting them know they must pick up the item and pay the difference..I was literally in line at the post office..She weighed the item and took it..I have never heard of anything so ridiculous..