If the characters are willing to talk present Old Marcus as the stranger most happy to see them after they return from an adventure; he will buy them a round, help them settle, and eagerly listen to tales of daring, heroics, and defeat with a sharp eye, quiet praise, and emotional support.
Old Marcus has a two-storey home a short walk from the inn and is well-liked by the people of the area.He is known for being friendly and open-handed with others. The only 'odd' thing is that he sends and receives many, many letters and occasionally has visitors from far lands.
If villagers are asked where his money comes from they speak of him buying and selling old books (true enough). If he is asked he responds vaguely about how knowledge is money.
How to introduce Old Marcus as a campaign resource: If the players don't think of this on their own either have Old Marcus ask about any old books they have found or have an NPC suggest they sell anything like that they find to Old Marcus. He haggles a bit but gives a fair price for any genuine work, especially written histories, journals, etc. Also, Old Marcus may show a shocking familiarity with some recovered object from deep in a tomb - when pressed he will simply respond he 'recognized it from a description in a book'.
What is Old Marcus up to?: Old Marcus, real name Marcuse of Greenhaven, is a former Religious Brother (see the Far Realms supplement for OSRIC for the details of Religious Brothers) and sage. He earns a living answering specific questions for various people and maintains an extensive correspondence with fellow sages, a handful of clerics and mages, and current, past, and potential clients.
Old Marcus information: Old Marcus is a 2nd level Religious Brother; h.p. 7, A.C. 9, can cast Protection from Evil once a day; may cure 1 h.p. once a day; may grant a second saving throw vs. poison once a day.
He has a +1 ring of protection (factored into his A.C., above) and a magical inkwell that never spills, dries up, or runs out (a gift from a former client).
His sage abilities are;
Major Field: Humankind
Special Categories: History, Languages, Demography
Minor Fields: Natural Philosophy, Supernatural and Unusual
How to roleplay Old Marcus: Old Marcus should be almost the opposite of the stuffy historian; he is a cheerful, outgoing, happy man who loves a good joke, a glass of wine, his pipe, and hearing a good tale. To him a stranger is just a friend he hasn't met yet. He will genuinely like any character willing to routinely share a drink and a story with him.
How he can help your campaign: While largely up to you, Old Marcus can buy old books of the party, translate old documents, etc. all the way up to researching curses, prophesies, and command words. He can provide clues, suggest possible locations for lost cities, etc. While he is careful to always charge a fair (i.e., full) fee if the players cultivate a friendship with him he will certainly help them first.