Taskrabbit and oDesk. Though over time the most common requests are usually standardized into products to reduce complexity for the buyer. For most buyers the open ended nature of a bounty system is intimidating.

The due diligence will be focused on these main areas: 1) product prototype, 2) team, 3) total addressable market and 4) operating model (i.e. use of proceeds).

Bill.com is fantastic.