Jeff we all know there's an obscene mark up and most of us understand it's a neccessary part of the business. What would be wrong with posting a copy of the invoice minus price and contact information? What we want (or at least some of us) is to know that the fish we're are considering buying are collected in an environmentally way. A store willing to go the extra distance in this area would have my business in a heart beat. All of this is moot, just shop at a place where you can trust the retailer. For me that's J&L Aquatics.
PS: I'm not sure which retailer you're working for
but I don't think you really want to be making threats of throwing people out for simply wanting to know where the fish came from. I know one place has threatened me (and then threatened to call the police when I disapproved of being threatened) and I've never shopped there again. I do go to laugh at the fire trap every once in a while but I've never bought a thing since.