Here's how it works. If you want data, you put in a request and optionally, a deadline and budget. A provider can then go find that data for you, maybe through scraping a difficult-to-parse website, and then post it online. You then have the option to purchase the tabular data.
There are three big humps to get over though for Data Marketplace to work.
The first is that there has to be people who are willing to buy data. There's so much emphasis nowadays on open government, transparency, and free APIs that the thought of actually paying money for data, to the average consumer, could feel a little weird.
The second thing is that the data has to be publicly available somewhere in some form. It seems that in most cases, you're not paying for the data so much as you are for someone to scrape a site and get the data in a machine-readable format. If data isn't openly available somewhere through an application, web forms, or whatever, then most likely it's behind a wall for a reason (e.g. privacy).
Finally, there has to be analysts, programmers, etc. who are able to get the data people want. Now there's no shortage of people who can do this. It's more about people who are willing to do this. Looking through some of the data requests that are already up, you'll see that a lot of them are very specific and aren't just a matter of scraping one site, but many sites, along with some analytical savvy. Not a lot of people are going to put in that effort unless the price is right. It then goes back to that first thing about what consumers are willing to pay.
This of course is in direct competition to Infochimps, who opened up their marketplace a few months ago, and sites like AggData, less so. It'll be interesting to see where they go with this in the next six months.
I think in the end, for these marketplaces to really thrive, there's going to have to be some kind of community that develops around the data, sort of like that of Freebase. There's only so much demand for Walmart, Target, and McDonald's locations. The first two by the way, are freely available here on FlowingData.