I experienced this on a world I was playing with friends,we have a librarian that sells 1 emerald for 1 bookshelf and 1 book for an emerald,so this means that:

-you could use 1 emerald to buy 1 bookshelf,break it and get 3books and then sell it for 1 emerald (3 in total).

That brings back to a bug from java edition which is the "Infinite Emeralds" but put in bedrock edition so please fix this this is too OP.