Used Bookstore in libraries?

Someone mentioned having a new library that also has a good used book store in the library.  I know a lot of libraries have on-going book sales but I wonder if there are libraries with full on book stores attached to them?  I wonder if that would bring in extra income for libraries (are libraries even allowed to run businesses out of them?).  I mean I know when I turn my books in for credit I don’t get nearly as much as I see the bookstore then selling the book for so it seems like it could make some money for libraries to go to other programs.  If the library were to give out credit for donations I can see them getting more books  and they could also sell books they need to weed out of their collection.  Granted you would need to hire more staff to man the bookstore rather than on-going book sales which are usually honor system that people leave 50 cents or whatever per book they pick up.

I know I for one would love to have a used book store near or attached to my library.  It would definitely save me a trip if I read a book that I decide I want my own copy of.