Exactly what I came in to say. Except, I’ve gotten in the habit of just remembering the page number and not having to mark the page at all, but if needed I’ll do as you do: dogear a paperback, use the dust jacket for a hardcover.
Folding the bottom corners is for remembering a favorite scene. 
(And if you check any of my many trashy romance novels, you’ll discover that a folded bottom corner usually means something racy is about to happen.)
I can juggle three books with the numbers in my head usually. The only time I forget numbers is when I’m interrupted from reading and lose my page. If I have time to commit it to memory then I’ve got it locked
Are the next few pages sticky? 
Re dust jackets, I pull 'em off and stick 'em on the top shelf while the book is being read. I reunite them later.
Re selling books, while I don’t think I have any serious moral qualms about doing it, I never actually have. Hmmm.