Some other things that havent been touched on.

I'd say it is an absolute must to have:

1. A good exotic vet
2. The funds to pay for the vet.
3. At home remedies and the know how to use them for minor things
4. A seperate quarantine room for sick animals.

Your business in the market relies heavily on your reputation. If you intentionally or accidently sell sick animals to people your reputation will suffer and so will your business.