The popular fish I see around me (in Chicago) are
zebra danios and on one rare occasion I saw some ocelot danios (bought those), I would rather see the different types of danios over zebras personally

I like ocelot and celestial pearl
hillstream loach
variety of angelfish types, scalare. I haven't seen altum or leopoldi...
gold, blue, dwarf, honey, pearl gourami
albino, bronze, panda, pepper, skunk cory
otos
blackwidow, rummynose, cardinal, neon tetra
kribs, german blue rams and german gold, bolivian ram
cherry, tiger, rosy barb
Harlequin Rasbora
a variety of colors for swordtails, mollies, platys and guppies
Boeseman rainbow, red rainbow
gold nugget, bristlenose, angelicus, clown pleco (and common but we all know most people can't house those properly...)
Bettas... I've never seen them in local stores, only big chain. Not saying you can't sell them though
I've seen a couple figure 8 puffers and a variety of synodontis but careful because some get big.
And as for cichlids... I honestly can't tell the difference
I've seen jack dempseys, parrots, oscars, convicts, green terrors, discus.
Do you plan to sell plants?
Java fern, java moss, moss balls, amazon swords, ludwigia, hornwort (lol), pennywort, wisteria, vals seem to be popular.