You are trying to sell the wrong thing.
Software is not scarce, the only way to sell it like it is scarce is by artificially making it so. That is what closed source software (or like you mentioned patents) do.
Making mony with GPL licensed code is not that hard, (many others are doing it), but you have to sell the thing that is truly scarce: Your time and knowledge.
Have them pay you to make it or improve it, or sell support.