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There are various marketplaces for buying and selling links between websites. Generally, the more PageRank your site has, the more you can charge for links.
However, Google does not look kindly on paid links unless they're market as "nofollow" (ie. the link tag looks something like <a rel="nofollow" href="http://...). Because nofollow links don't pass any PageRank to the link target page, you'll have a hard time selling nofollow links (many if not most people buying links buy them for the PageRank value).
If Google discovers that you're selling non-nofollow links, they may remove you from their index. For this reason if no other, I recommend against selling links.
* Amazon.com best sellers displayed using CaRP Evolution with expanding reviews powered by CaRP's "Expander" plugin.