There are a lot of people marketing software libraries, components, add-ons, etc. But the overall market seems pretty small. And small scale developers seem suprisingly cheap- they often opt to develop their own solution to a problem rather than buy a library component that costs only $50.
Check out the ASP (Association of Shareware Professionals) for more information. Many of their members develop components and libraries and virtually all of them use these tools.
Targetted marketing of these components can be difficult. There is a web site Component Source that specializes in selling tools like this. You can list and sell your product with them for a fee.