The exchange isn't even, it's a really bad exchange rate. If you stood in front of him you need 30 marks to buy enough materials for 25 reputation and only 5 marks back. I already showed the math earlier. To start with you have to begin with materials or marks. 60 marks or 100 materials to have enough to trade to get back enough for a second exchange. As I pointed out earlier you are also restricted by how many marks you can have and gain.