Very often in the fish departments of larger full line stores—and almost always in stores that are fish only—there will be one or two tanks devoted entirely to brackish-water fish. This makes for an excellent opportunity to sell your existing customers that second (or third or fourth) aquarium setup, just to keep brackish water fish.
The tank setup for brackish water fish is pretty much the same as for freshwater fish, only with the addition of salt and a hydrometer to measure salinity. Your customers will have much better luck with a brackish tank if they use any of the good marine salt mixes for the salt in the tank. Kosher rock salt can be used, but a good marine salt is much better. It is also a good idea for them to buy an inexpensive little glass hydrometer to measure salinity.
What has really led to the current success of brackish water tanks is the many brackish water fish being raised commercially. This started with puffers. For some reason, an ichthyology class in Florida decided to raise puffers—and they were very successful at it. Now there are quite a few different commercially raised puffers available, mostly from the Far East. It is important to be sure the species of puffer you are carrying is one that requires brackish water, as not all do. Other commercially raised fish now include scats and monos, all of which used to be caught wild and are now raised in the Far East. Also, unfortunately, they raise Arius catfish, which is sold at around a 4-inch size. I say unfortunately because these fish grow fast, become very large and as they mature require straight marine conditions. One fish that does great in brackish water is the molly, especially the large sailfin varieties.
Brackish setups seem to do best with fine marine sand with some coral sand mixed in. Live plants can be problematic, and if you and your customers want to try them I would start with vallisnerias and maybe some Java ferns. One nice thing about brackish tanks is that sea shells and coral rock make excellent decorations.