IMPORTANT PLEASE READ
Here are the details. To make a long story short, they have subscribed to a service that finds certain keywords in links ("Buy" and "free" are two that have been noticed) and causes those links to redirect to certain commercial sites, like Amazon and eBay.
So far it seems there is a long--but finite list of links that trigger this redirection--more information at the post I linked to up above.
There's a way to opt out, but it's a little weird. Again, it's laid out for you at the above link.
Seriously not cool LJ. Fail.