Jukeboxes, like you see in bars and restaurants, do have to have licenses because they are in public playing to a public audience. If you're renting equipment to a private party, this is no different than the service you provide (as far as copyright is concerned).
Yes but if you did actually want to be legal you'd have to purchase multiple copies of your library, wouldn't you? Realizing of course that most DJs (me included) would just duplicate and use the same library in multiple locations.