Try to use your knowledge to trade with him

Marcos is a cunning trader... He surely knows of most trade routs, but you have just happened to read about the journeys of an brave explorer, who dared to wander south of the Sahara by the sea and discovered great riches and civilisations there. Even made a map of it in a book, that you have in your room just here in the castle.

Lily: How about I tell you of new ways to earn more money?

Marcos: How could you do that?

Lily: I have a few connections... Heard and read a few things about the world...

Marcos: What things? What we do not know about in Constantinople, that hardly exists!

Lily: What if I tell you, there are still lands we haven't properly discovered?

Marcos: Sure, there are, but only wastelands!

Lily: And if I told you somebody could traverse the Sahara?

Marcos: Hm... I have heard of tales some nomads could do that and the legends say, there are rich empires there, enriching themselves on salt and goldmines... But these are pure speculations and fairytales. Even if they are true, we have no way of traversing the Sahara, it would cost us thousands of lives!

Lily: I didn't say walk across it... Sail around it instead!

Marcos: But that cannot be done...!

Lily: It could be... Look, I have a book, containing a map of a journey made there, not long ago. If you can offer something for this information, I might be willing to part with it. Deal?

Marcos: Lady Lily, your cunning is one I underestimated... I once read of such journeys done by the Phoenicians, but never dared to imagine them to be true... If I could travel there to check those riches...! Right, make a deal. I'll offer you 500 gold for that book... Agreed?

Lily: Hm... Fine, I can't travel there myself anyway... Give me the money and I'll hand you the book.

You make the trade and part ways, both of you satisfied with your gains.