You can play gwent with him though he doesn't give any unique cards.
Sometime prior to 1270, Jacques de Aldersberg stopped by, giving Marcus a book with a letter inside and instructing the shopkeeper to give it to the witcher Geralt if the witcher ever visited the shop. The book proceeded to sit around the shop for years, with Marcus not believing such a person would ever drop by.
If Geralt visits: One day in 1272, Geralt did visit much to Marcus' surprise. Having not forgotten about the book, he then informed Geralt about it, though he forgot where he placed it. Thankfully the witcher was able to find the book in question.
If Geralt brings and sells him the painting: Geralt learned about Marcus being a fierce admirer of van Rogh and proceeded to bring him "Starry Night over the Pontar". Marcus, excited over owning it, readily paid for the painting and hung it up in his store.
- The character is a reference to the author of the Witcher 3 strategy guide, written by David S. J. Hodgson.
- He's credited as the author of the real book, A Fractured Land: Tales of the Northern Realms, which is another witcher lore book written by David S. Hodgson.