I own a few websites that I would like to link together (probably just a text link at the bottom of every page). I was wondering, would this have any negative effect with Google as it may appear that I have been selling links on the sites?
You should consider, that Google wants pages, that are made for human beings, not for search engines. Just placing the link for the sake of it would be a reason for a penalty. If it is done now - nobody knows, but when doing link building, you have to think long term and many favourite methods have seen a decline in effectiveness lately. It's not like Google is unchanging. Your are better off doing quality links from within page content.
If you own all the domains for the sites then Google is unlikely to see it as selling links. It may see it as a link farm, however, and anything that may appear to be manipulating the SERPs is very likely to be penalized in some way. In my personal experience (on a couple of sites very recently), suddenly adding footer links pointing to another website is likely to initially negatively impact your SERPs for the site linking out in particular. I would experiment doing it on one site and without altering any other factors, watch the SERPs for both sites. Don't react too quickly to a drop, site and wait a little to see if things improve. Stability is increasingly important, so a sudden spike in the growth trend of links to a site can raise a red flag.
SE's like to see relevant sites and content. If your sites have different ones preferably not to link them.
I think its okay in moderation... but if you are using at as link building strategy between dozens or hundreds of websites then Google will spot that...
If you use rel="nofollow" the link won't pass any link juice. So the only benefit of having the link on your site will be if someone clicks it. Instead of putting a link from every page of every site to every other site pick an important page or two that are related between your sites an place a link there. Only link between sites that have related content. Put the link somewhere in the page where a user will see it and click on it because the link is useful. You'll be o.k. with the search engines this way...and you'll increase the referral traffic and your SERPs.
Google does not approve of site wide links so you may get penalized for doing this. I think freelistfool has a great idea.
You can place links to your site anywhere you want. If google values it as a true link is another story. Google will not penalize your websites for incomming links. If this where true any of us could completely destroy our competitors ranking by simply linking to their sites from undesirable locations. Anything else is just people spouting something they have heard and never actually tested.