So you are making a DLL, presumably for developers to use, and want some kind of copy protection?
My first thought is "forget it".
As a buyer of many libraries (or actually persuader of boss to buy libraries) I can safely say that I would never even consider buying any library that doesn't come with full source code. A binary only DLL that "phones home" or something like that is completely out of the question. Always.
Reading the first post again, you are planning to sell your DLL for about $10-$20. Have you really thought this through? How many are you likely to sell? If the library is useful, you can sell it for many times that price. My boss easily pays $1000 for a library without blinking, if I say I need it.