Do not resell the design for more than you paid for it without additional art (that you commissioned) attached. Gifted art does not count towards the value of the resale.

I don't mind minor changes to the design such as new hairdo or a gender swap, but please don't turn a white man into a black woman, that's not a natural change that would occur. Simplifying or going more elaborate on design details is fine as long as it still looks like the same design.