I worked on a project like this a while back. The heavy regulations eventually broke the whole concept. We had built a system for startups to pitch their businesses, and the investors would have been limited to accredited investors (those with > $1M or who make > $200K/year for two years). They would have bid for startups in a Dutch auction format, like the way Google went public.
However, to do this you need to be not just a startup, but an NASD certified "broker dealer." This imposes a couple things that software startups aren't good at:
1. You need $500K in the bank, just because of regulations. This means you need to raise much more than usual.
2. Several founders/early employees need to study for and pass Series 7 exams, which is what you need if you're a stockbroker. At least one person also needs to study for a similar test for brokerage management. Coding + learning to be a stock brokerage was not easy!
3. The associated legal costs were extremely high.
Kickstarter of course avoids all this by not dealing with any transfer of ownership so they don't need to worry about the government. They just sell access to creative folks, which is a good idea.
PS - Angel investors and VCs hate this concept, which makes it much harder to raise money. We brought it in front of all the biggies and were well connected, and each one argued about the personal value of their involvement, which may or may not be true.