In the Classifieds it is easy. Log in with your user name and password.
When you see someone interesting that you want to save for later as a follow-up just click "save" at the bottom of the classified ad.
http://forum.myredbook.com/dcforum2/User_files2/k2v1l0w8rwj617wv.jpg
Later, whenever you want to check for updates to their prices or anything else, just go to the top of the Classifieds where you will see the link for "favorites" and everyone you saved will be in there. You do have to log in for this. Or did you have something else in mind?

http://forum.myredbook.com/dcforum2/User_files2/20r2dwg46zcwv28w.jpg
You could always bookmark a Personal Ad but since those change and are designed for same day availability (primarily) where the Classifieds are more of a permanent and searchable thing.
The other idea that I used to do is go into "user menu", then "scratch pad" and post whatever information or the entire ad into a new entry.
Classifieds sounds like your best bet and more in line with what you want to accomplish in an easier manner.