Mercari Sold an item using Mercari's shipping lablel, and they stole my money San Francisco California

In December of 2015 I sold a pair of Vibram 5 finger shoes to a customer on Mercari. I used the shipping label that mercari provided me and shipped the item.  I  received notification that the item was received, and the had customer rated me and everything.  Only to find that  on 4/7/16 that I get a message stating that a Prohibited transaction was detected, and they remove my funds out of my account stating until I can provide proof of shipment.  

I gathered all the necessary information the label, the messages stating that the transaction was completed, and they tell me that my item could not be tracked as if I never mailed the item.  They gave me the run around by telling me to contact my local post office, and post office tells me that the tracking number on the label does not come up at all.  I was advised by the postal service customer service representative the after a certain amount of days that the numbers are recycled and used again.  

I then contact Mercari to advise them of this, and they claim that they are able to track packages futher than that.  So, now I am here without my product, and without my money that Mercari has stolen from me.  I have requested numerous times for someone to personally contact me regarding this matter because I keep getting improper responses and am not getting anywhere with resolving this issue.  And they never called and I requested the corporate information which I am still waiting on.   My advice is to ship the item your self.  The one item that I did ship myself I had no problems.