Our approach is to start really small - we only need a small number of hackers and clients to get started. This allows to be in a similar situation as an small agency - we don't need to match the audience of elance/vworker to make it work. The human/case by case approach requires more work for us, for a lower cut than what an agency would take, but with a different goal in mind for scaling.
We could actually have done the first batch by ourselves, with just clients and hackers that we know and are interested. But we thought it would work as a better validation to give a chance to HN readers to join - we're more likely to get a more diverse set of situations this way.
The advantage for those who will be in the first batch is the extra care and attention we'll dedicate on making it work for them. We'll all be learning from the experience - aside from guaranteeing a place on Hacker Trade in the future, it's also a good opportunity to shape what Hacker Trade will be, and ensure that you get what you need from it.