There was one series I did re-buy in ebook. What happened was that I had some of them in paper but not all because some I had originally read from the library and some I had gotten rid of and/or lost in several years worth of moves. And then one night I started re-reading them, finished one book and the next one was one of the missing ones, and I got so annoyed
So I waited for a good sale (this was in the pre-agency days) and bought all of them. Now I buy the new ones as they come out.