I think specialty shops run better than very large and general shops. So i would suggest being a herp specialty possibly with salt water, or tropical fish, with only one or two tanks with hamsters or gerbles. You may have to start with a general shop then focus your shop into an area. But that is just my opinion, i am sure others will say general all the way. top sellers when i was at a pet store:
snakes (maybe not exaclty this order):
1. Ball pythons
2. boas
3. corns
4. Burms
5. king snakes/milks
lizards (stay away from iguanas! too many problems including mites):
1. leo's
2. beardies
3. anoles
4. uromastyxs
5. tokays/various other climbing geckos
6. savs/monitors
amphibs:
1. african claws
2. fire bellies
3. pixie or pacmans (abino pacs are popular)
4. White's tree frog
5. green tree frogs
mammals (stay away from cats and dogs, plenty for adoption)
1. hamsters
2. gerbels
3. guinie pigs
4. rabbits
5. chinchillas
6. feeder/fancy rats and mice
Birds:
1. parakeets
2. cockatiels
3. amazons
4. cockatails
fish:
1. cichlids
2. neons
3. goldfish
4. mollies
5. sword tails
6. plecos
7. guppies
8. feeder fish (golds, tuffies, guppies)
Feeder inverts, frozen rats and mice.
breed your own stuff, people like that you are selling all your own animals and they can know where they're comming from
just what went at my store
~mike