The Stamp in Graveyard Keeper is needed to sell meat at the inn. If you have the stamps, you can mark flesh cut from bodies, so that the NPCs are more than happy to buy it. You can use the stamp multiple times, but it's quite difficult to obtain.
You can get the stamp in Royal services - near the church. It costs 50 silver coins.
You can also get the stamp by completing a mission for Snake. After reaching 30 friendship points, he will just come to you and give you the stamp.