The only thing Kretzschmar has for sale is the final scrap for the U.S. Army Uniform making his shop virtually useless for the vast majority of the game except as a place to sell skins and other items. He also sells a scrap for the PS3-exclusive Walton's Gang Outfit, but that is also available at the tailor in Thieves' Landing.
He has a strange quirk, possibly a glitch. When you attack him and pull your gun out, he will start shooting at you, causing him to be shot by the law. However, it takes about 50 shots for him to die. He shares this glitch with Herbert Moon, the shopkeeper in Armadillo, Jeb Murphy, the gunsmith in Armadillo, and Alejandro Duarte, the gunsmith in Escalera.
In the third official trailer for Red Dead Redemption II, a short scene takes place on the docks of Blackwater, and Kretzschmar's shop can be seen in the background with his name on the building, implying that he may return for the second game.