I'd use bitcoin myself: lower transaction costs, make micropayments possible, even allow the tracking of coins if they should get usurped. Lastly term the investments as tips rather than investments. Then make everything transparent including the way coins are spent by the startup, revenue received and returns as "thank yous" to tippers. Have a limit to how much a startup can receive.. a certain proportion of tippers need to approve their initial tip to the startup in order for it to raise more funds: build a trust quotient. The startup can game the site by tipping itself to build that trust? Have people send in a copy of their ID and attach it to each user.

other ideas: don't even have the startups register as businesses. Have them only accept bitcoins. Make the site as transparent as possible: who has given to what and when? Have all revenue and expenditure by startups go through the main site for tracking, and the main site can collect a fee for it as it passes through. The main site pays all the taxes.