Entering the prebonsai market is easy. Actually selling decent prebonsai is the problem. We need more prebonsai nurseries, but we need good prebonsai nurseries.
Growing prebonsai (that is really pre-bonsai and not just seedlings) isn't as easy as just plunking seeds or saplings in the ground and waiting. Prebonsai are usually pruned top and bottom over the years with an eye to producing the basic trunk needed for a bonsai. That pruning requires some knowledge of bonsai to produce.
The problem is that potential bonsai pruned incorrectly or inadequately as saplings grow into trees that aren't really great for bonsai. The mistakes at the beginning can follow the tree all the way through its life and your mistakes will take a toll on the money you hope to make.
People willing to make the investment in usually more expensive "prebonsai" stock are knowledgeable enough to know what they're looking at. They can usually tell what's worth the money.
It's a cold hard fact that current sellers know well, which is why there aren't many people selling quality prebonsai material.
Before you start a prebonsai nursery, you're going to have to get some experience growing and caring for actual bonsai, I'm afraid.
Bill Valavanis at International Bonsai Arboretum is an excellent example of a notable prebonsai stock seller.
http://www.internationalbonsai.com/...g/bareroot.html