I think Matt Cutts said you should use no follow for paid links so you don't pass linkjuice. In the past I have had two sites with decent PR and that ranked well. For both sites, I had someone email me and want to buy a link, so I gave them a link with nofollow. For both sites, within two months the sites had lost PR and lost ranking. The only commonality was that I added a "nofollow" link. The target site wasn't a spammy site or anything. Do you guys use nofollow when selling links to advertisers?