The links need to be on topic or else you'll both get hurt. If you have a site about cars but you're not linking to car-related topics, Google will assume something fishy is going on.
are you sure about that? I'm not saying you're not correct, but I'm sure this is a poorly defined area.
And there are absolutely seo firms with automated crawlers/criteria looking to buy links. I'm sure there's some pretty interesting variants on how useful and succesful these are.
Now, I wouldn't sell links from my main site (well, not unless I got some obscene amount of cash). But that being said, if you're going to sell links, I'd suggest being a bit smarter than just 'selling links'. I'd be doing something to integrate the link into the page so that a manual review doesn't trigger a smackdown.
As for what you have to lose, I'm told you risk things like penalties for your rankings (perhaps more so than the risk of a penalty for the people buying the link). What if google hits a hidden switch that says your site no longer passes any juice?