A WebmasterWorld has discussion around a topic I don't think I ever covered... How does one sell links on their web site without getting penalized? Of course, you can nofollow the links but what about not nofollowing them?
First bet is to not do what we do here, because we are penalized. :)
(1) Make sure to always control the link text and URL manually (2) Only link to relevant sites (3) Link to sites that don't seem banned or sketchy (4) Do long term link sales, so it is less of a rotating ad network (5) Site wide foot links or side bar links are a red flag (6) Don't advertise the links are for sale
I think I can go on and on, so feel free to add your own or discussion.
Forum discussion at WebmasterWorld.