Why don't you take fees to support further development?
If Alice and Bob were to trade drugs on my website, and I take fees for that commerce, you can bet that law enforcement is knocking on my door very soon.
Craigslist took down their personals and they don't make any money off of it.
Edit: Additionally, the aim is to be an open protocol for trade and not just a product. Fees for using an open protocol don't make much sense.