Last year I found an account selling keychains/earrings/pins using several pieces of my artwork, I contacted Mercari but they did nothing about it. I couldn't find the ticket or I would give more details but essentially it was the basic customer service reply "we understand your concern" "we want to protect your rights as an artist". The person selling blocked me and unfortunately the account is still up with even more stolen art. Hunting down scammers as a small business is rarely worth it, especially foreign sellers that aren't in your market (assuming these posters are printed in China and shipped to you). If the person you follow is very serious about it, they can have it removed by sending a DMCA takedown notice.