From what I have read and experienced the best marketplace startups start by seeding the supply side with home grown information, content, products, etc. and then generate some success stories with those in order to attract more supply. I'm a believer that you do not "tax" the supply side until you have some critical mass. There are many ways to go about this and it can be niche specific. Transaction businesses work when you already have a fairly good transaction volume, so that's going to be your goal. Happy to discuss further.