Ask HN: How do I structure a reseller program for my webapp?

I have been recently approached by someone, who wants to be a reseller for my webapp, blogVault.

One of the things he mentioned was that, he wanted to co-brand it, so that he can mark up and charge his clients. Now I had typically imagined it to be a situation where I had a relationship with the customer directly. What are the ways in which I can approach this situation? Is there a limit I should set on the markup this person can add?

One other concern I have here is that, there will be certain people, who will actually be paying more for the service, just because they did not directly visit my site. Can this be a problem?