I don't think the issue here is you making a dent in the snail population, but instead an issue of transporting a disease that could wreak havoc in an ecosystem elsewhere. The snails in your area may not be affected by something but if you ship them somewhere and that person puts them in their local waters then your pretty much helping them spread that disease to an area previously unaffected by it.
VHS, viral hemorrhagic septicemia, is a disease that is spreading right now and needs to be stopped, and is also known to exist in New York. This is killing fish in large numbers and spreading partly from people transporting fish from one body of water to another. It initially came here from Europe from boats carrying water in their ballasts across the ocean and then releasing that water once they came into the great lakes region.
This may not kill these snails but that doesn't mean that they are unable to carry VHS. What I'm doing with my tank is not aiding in the spread of VHS, because I'm not releasing fish back into the wild after I put them in my tank and I'm not shipping these fish to areas that are not having these issues with VHS. I'm keeping these for my own enjoyment and thats it.
I'm not necessarily saying that you would be spreading VHS, but I'm just using this as an example to show why doing something like this isn't really a great idea. I don't think anyone would have a problem with you collecting fish and snails from your own waters for a native saltwater tank, but taking these fish, snails, etc. from these waters and trying to sell them for a quick buck isn't a great idea.
I'm done with my rant on this.