Ive seen secondmarket.com and sharespost.com but they seem to be for institutional investors and not very straight forward.
I'm talking about investing sums of around $500-10000.
You mean something like circleup.com? Do keep in mind that the ink on the JOBS act (which regulates small investments) is barely dry, I think it passes just yesterday!
I'm actually outside of America ATM, so I havent noticed.
I (and others on HN, I'm sure) would be interested in pitching you, if you can invest with a clear, reasonable, and legally sound seed funding contract/agreement.