How to help grow? Well, it looks like you've already got link sharing for facebook and twitter - how's that working? Seems that would be a great way to get more exposure and people interested. You've got the sbux gift card, you've got bonus points for referral...how long have you been up and running??? It seems to me you've done a lot of things right! Oh, and the site looks great too!
Have you reached out to those who have listed on your site to get their feedback about the experience? Or, have you gone to garage sales or the like and asked people what it would take to get them to use your product?
How about this: Have you scanned listings on craigslist, reached out to the seller, and offered them free listings to give it a shot and get feedback? Maybe target a specific area, like cars, or iDevices, and really hit it hard.
How about mobile apps? Native iOS or Android app that initializes the camera, records, allows me to provide the meta data (description/price), gets from the phone my location to save me that hassle, and then post it all from the app? That would be pretty convenient!
As I was going through your site I thought of a somewhat ridiculous concept, but I figured as an entrepreneur you would be open to any possibility. I guess I'm just one of those people who love brainstorming about exciting possibilities, even if some of my ideas are completely infeasible!
As I was going through a few videos, I noticed varying levels of production quality and video review styles among sellers. Perhaps a way to homogenize the experience is to offer individuals a service where they send you their stuff and your company can handle the video reviews (maybe add a distinctive flair to them) or even do a little product testing in addition to actually selling their goods for them. The premise is to completely take the item off the seller's hand and not have him/her worry about it.
I can probably list a dozen problems with this idea off the top of my head (and maybe there are already services like this??), but hey, you never know what may change down the road!
Anyway, best of luck with your venture!
http://123exchanges.com/default.aspx
Didn't you post this here some weeks ago? If memory serves me right, you've made some (significant?) changes to the home page payout since then - I think it is much better now.
All the best.