There was a thread about this not too long ago.
My personal opinion is that its definitely possible. I mean look at the online dealers right now. They did it so can you.
Here's the thing:
As I see it the initial investment will be large. You'll want to set up a way to breed many different species of tarantula, scorpions, or reptiles. Then you need the space for the babies. Then you need a web domain name. Then you need containers, accessories, space, and an ability to keep a good temperature. Maybe even afford an assistant or two.
That's only a small part because I'm sure there are legal issues involved including permits. Then you have to either buy breeding groups or catch them in the wild. And then there's establishing a customer base and shipping.
But its not undoable and it is probably a way to make a living if you're smart about it and really care about your work.
It seems to me that opening an exotic pet shop that also ships and breeds at the store is a good idea too. But that's just me. ^_^