In other words, you want to start your own business.
What specific type of people do you want to sell to - anyone who wants a guppy, or do you want to sell to fanciers? How many types of guppies are there?
What is your return/refund policy? If you sell a guppy and it dies, will you refund their money?
What diseases do they get?
Are you going to ship them cross country? If so, do you know how to insure they are still alive when they reach their destination?
How are you planning on advertising?
How are you going to compete with pet stores?
Can you tell the sex of a BABY guppy (which doesn't have the color)?