Depends on where you are and how much they enforce federal laws. And also how they're being sold. Companies that are considered to be selling periphernalia for illlegal purposes can and sometimes do get busted. People who are caught with marijuana often get their pipes or bongs taken from them. But if you're selling a water pipe in a tobacco shop say, they're not going to do anything. Recently some head shops in Cali were raided and shut down because someone decided that having High Times magazines near bongs was probable cause to enforce anti parephernaila laws. If you have them and you're not busted for anything else you'll be okay, and if you're selling them and not stating that they're for pot you're okay. And if you sell them from outside the US you're okay.
I was talking about the federal government, I'm not too familiar with state by state laws, I know it's not illegal in Illinois or Wisconsin though.
dude we blow glass and walk around with it in back packs or at the shop we've had cops come in search us and walk out but never said we had to get rid of n e thing i think they thought we were slangin out the shop or somethin but we always sell glass, even at the flea marts we set up a show case, maybe cuz im from california and laws are less strict but we have mad bongs or water pipes