It depends. I store all my ebooks in cloud storage so they don't take up memory on my tablet. Text takes up very little memory, so you can keep a lot of books on most devices; I just prefer not to do it.
Technically, I don't own anything. I am a licensed user and have access to the ebook--maybe forever, but maybe not. Sometimes t
he seller takes a book back and sometimes you have a book that doesn't survive a technology update (I am still bitter about a cookbook).
If you aren't too particular about legalities, however, you can always convert your ebooks into a different format and keep them forever. Not that I have ever done this

, but I've heard that it's something people do.
Ebooks are nearly always cheaper than print; I have never paid more than $5 for an ebook and I've done that only a couple of times when I really wanted a book as soon as it came out. I usually wait until a book is under two dollars, which typically doesn't take very long.
So yeah, you can't sell them or donate them or anything, but if you want to get rid of a book, it's just a tap of a button.