I sell footer links for my word press website themes. However, I am at a loss to find a reliable way to protect the links for my themes.
You can't protect those links. But what you have to do is just play the numbers. Most people who use these have no clue on how to remove the links. I wouldn't loose too much sleep over it. It's just how wordpress is.
As TextLinkGuy said, you can just go with the numbers as lots of people will either not bother to remove it or will respect your work and leave it there to give you credit. You can use PHP to encrypt the footer, but that won't stop anyone who wants it out and knows how to use Google. The people who are too lazy to use Google or just get confused by it, while still not wanting your links on their page, will probably just end up using a different theme.
what does your mean how does protect no sell link then no link in your footer. tell me what exact you want
where there is a will there is always a way . I think the above posters are right just play a numbers game. Why not ask for a link else where on the site if the footer link is removed am sure those using the template will be more than happy to give you one.
Yes, you can easily protect your links in footer by encrypting with base64 encryption but always make sure to encrypt the complete footer code not just the links. If you just encrypt the links, people are smart enough to recognize the encrypted code and will remove it but if complete footer is encrypted there is no choice with them other than to have links in footer.