There is now a new version of phpLD that allows you to run a site that allows people to bid for position on their links. While this is still new product, it seems to be working flawlessly for people that have tried it so far, and it even seems people are making money from it already.

More Info: Here

This is a free download, but we ask that you retain a link back to phpLD, or pay $50 for link removal.

Here is a nice template for it too:
More Info here