Your point that OP would have been way better of if they did let their agent handle it from the beginning was made quite clear already:

> Looking back, I can see that I should, at this point, have handed the conversation over to my agent, Charlie, who negotiates my book deals with publishers, and who is a complete darling, and good at this sort of stuff. But I got stuck. I think I thought that bringing in my agent would potentially mess up this pure art relationship. Maybe he would be too ruthless, and it would all fall apart! Of course, now I can see that it would have had the opposite effect; Markus and I could have talked art, and Carl and Charlie could have talked money, and everyone would have been much happier.