With a lot of theft occurring at shows I am not a big fan of open booth to the back wall designs. Too many books disappear.
Asking to come back to look is generally respectful as far as I'm concerned. Some get offended if I don't let them back to look.
If I know someone a long time and have seen them handle books I really don't mind them taking a book out.
However, I have witnessed some very brutal book handling at my booth and if you just so happened to be walking by at the time you would be witnessing a "Bad bob" moment. I can understand kids being a little rough but frankly am not sure why adults feel it's necessary to bend my books back so that they can see what they are looking at. Or if they can't put the book back into the box they feel compelled to stuff it in.
I have witnessed some books falling off my shelf from a helper I know very well. Needless to say the slow motion reactions from others around him were priceless and thankfully no book was damaged. That's why it pays to have a bigger board around your books in case of things like this. Dent the board, not the book (Remember, Hotflips has you covered )