This is an area that I'm still having trouble with. It's pretty difficult when you have an idea that is workable in more than one market - which one do you choose?
The "Wantsy" concept is similar to "wanted ads". A user posts what they "want" and someone else provides it. If you have a system where the seller's offers are visible, it creates a reverse-auction setup with sellers trying to bid lower than the other. That's it - it's a simple concept, yet it seems so unexplored. Sure you can find it on a classifieds site, but the concept is really underdeveloped with little focus. There isn't a dedicated solution for users "wanting" things.
Which market should this be applied to? The existing wanted ads market? The real-estate market? The advertising market?
Existing wanted ads market:
There are over 1 million wanted ads posted to Craigslist each year. The focus on Craigslist simply isn't there - you're stuck locally, categories are non-existent, and there's not attempt at a reverse-auction setting. If you make wanted ads the "meat" of the site and focus only on that aspect, I think you could win over a lot of wanted-ad-users.
A decent percentage of wanted ads are comprised of people seeking "real-estate". This is technically included in the above market, but I'm not sure whether it should be the sole focus. Servicing a clear niche market like real-estate (on it's own) would make marketing a lot easier.
The advertising market:
The advertising market is covered well by services like AdBrite, but what if the model is flipped? What if it's the advertisers that list their need, and the advertising distributors that seek out the advertisers? It's typically the other way around. As a website owner, I would love to see who's interested in advertising on my site. As an advertiser, I would love to have distributors bid down on my costs - besides, advertising isn't an "impulse" item.