VHDL is often hard for software engineers to understand and learn. The whole concept of signals and variables as well as concurrency is all a bit strange to softies. Hardware engineers find it easier to grasp, but are not always the most talented at writing elegant, well commented code.
As for buying and selling code, it really depends on what it is and why you want to buy and sell it. Proper code that is used in real products costs thousands and is sold by a few compnanies. If it is for learning puropses, there are plenty of VHDL and more generic HDL forums and websites that will sort you out with bits of code for simple things for nothing.