Mercari is a great place to find deals, but it is a tricky site. There is many times where there are too good to be try deals and I would recommend stay away from those. If you buy something that is selling at a reasonable discount you should have no problem. Also, make sure you are buying from a seller that has at least a few ratings and you will be just fine. Normally the scammers are the ones have just created an account and are giving something that is expensive for a very low price.
Thank you for your feedback! We also suggest reaching out to your seller before a purchase and asking as many questions as possible to gauge seller behavior. Because rating a seller releases funds, we also recommend not rating your seller until you are satisfied with your purchase. Thanks for the helping keep Mercari safe and friendly! -DE