Most likely, all of the sick herps are getting the others sick.
The ones that look healthy could be incubating the illness, which means if any of them are sold, the customer will bring home an animal that may get sick and die, and infect any other reptiles they have.
The ONLY responsible (and remotely ethical) thing to do is to pull them all into the back room, and do not get more in until the issue is solved.
This sounds like one of the scariest illnesses I've heard of so far--affecting turtles, lizards and snakes. Paramyxo is generally only found in snakes, so it's less likely to be that...but that doesn't leave any obvious known viruses. This could be something new, or little known, if it is viral. With the sudden onset and so many deaths, it sure sounds likely.