Advice on selling source code so it can be used to create a white label site

I have a product that I created originally as a SaaS solution, but since I was in a market filled with better funded competitors, I decided to change direction and target software developers directly, selling them a system they can easily integrate into their own applications.

I have had a few people contact me recently asking if they can purchase a software license and then use the software to create their own white label solution. Whilst they wouldn't directly compete with me (I'm planning on shutting off my SaaS solution), I can't help but feel they are taking advantage of my source code.

I offer two three packages. Free, Unlimited and Unlimited + Source, priced $0, $999 and $1499, respectively.

I could modify my SaaS solution to offer white labeling, but I'd be entering into a space with some established competitors and I don't have the time or will to maintain a full SaaS solution like that. It was another one of the reasons I decided to sell the system, rather than offer it as SaaS.

Do you a) think it's appropriate for people to use a source code like that e.g. license it and build a not-dissimilar product and b) is $1500 a fair price?

Thanks