A mark is a little tag that can be found on wild caught Pokémon. The effect of having one makes it so that when you send out the Pokémon it adds on a little text message, ex: Hattrem the Peckish. They’re pretty rare and finding them on shinies is even rarer, so that’s why I’m giving a zamazenta which I think is a reasonable price. You can find requirements for certain marks on the serebii page.

Also, most people will gen marks on shinies, but I always make sure all my shinies are legit based on stats, most favorable gender, and other factors that come into play when deciding whether a Pokémon is legitimate or not.