My 2c after watching the video: I don't understand the instant negotiation. Is there a limit on the number of times I can submit a bid? If not, can't I just start really low and then increment by $1 or $2 until I meet the minimum? If there is a limit, what's stopping me from setting up multiple accounts so that I can discover the minimum, then bid that amount on my legit account? Since this seems to be a major differentiation point, I hope you've ensured there is no easy way to cheat the system ;)
Also, how do people pay each other? If you're relying on people to exchange money privately, how are you going to handle disputes?
One more thing about the video: your accent. I'd try to get someone without an accent, probably a woman. I think this would appeal more to your target audience.
How many people do you have working on this besides you?
As far as the Onista site goes, you've done a fantastic job with it. There seems to do and explore and the interface is intuitive and clean.
Wish you the best of luck in the coming weeks!
Matt