You can easily sell off RSS ads if you are using self hosted Wordpress by Feedvertising program of Text Link Ads or can directly sell some text links in the your post bottom if you have got a wide RSS audience with a plugin.
Blogger too has got a similar feature but involves a game playing with it.You need to go to Settings > Site Feed > Feed Item footer .And add the desired HTML in the Feed Item footer text area.Remember to redirect your feed to feedburner, enable post pages and a full site feed.
You can also add Adsense for Feeds code here if you are participating in it.Remember Adsense for content and Referrals ads will not show up using this trick.
This method can also be used as a copyrights measure
I have added a copyright message which includes a link back to this blog from where Technorati reactions tracker could be used.
The copyright text is shown up in feed.
And the the person who used my site feed to put the exact content is caught using the technorati tracker.I have visited and flagged the blog.Hope, Blogger tem takes an action soon so that visitors get information from original source.