Mercari complaint: Accepting false returns allowing scammers to steal

There has been five different lots then I have sent out and they have approved the return the one was for Jewelry it was for 15 pounds they the buyers first said that it got stolen out of the mailbox then they asked for a return I asked Mercari how can he file a return when they said it was stolen then the another incident I sold some earrings they asked for a return they approved it they don’t ask no questions the last time it was a black heritage postage stamp book the complete series it has 6 stamps A in every series has 6 stamps they come with a certificate of authenticity as well and they approve the return and when I get my items back I get nothing the first time I sent 15 pounds out I got 2 pounds back how is that even possible iMessage them and they respond with item was not described properly yes it was photos taken I follow all policies this is getting ridiculous and costing a lot of