The buying and selling of virtual goods is already proving to be a viable business model for companies like Zynga and Playdom, but today a new study is revealing that players are increasingly selling and reselling virtual goods among each other.

The study which was sponsored by PlaySpan and VGMarket, sampled 2,425 customers across PlaySpan's marketplace.  PlaySpan provides virtual payment solutions and currency for around 1,000 online games which span Facebook, MySpace and its own site.  

And these players are making actual US dollars.  31% of the respondents said they had sold digital currency in the last 12 months, earning a median of $22.  

Narrowing it down to genre, one out of two sellers made a sale in a social networking game and earned a median of $50.  While one out of four users made a sale in a free-to-play game, earning a median of $98.

Some other interesting stats from the study showed that 89% of the sellers were male, averaged an age of 21, and had a household income of $30,000.  Eric Hartness, Chief Marketing Officer of PlaySpan believes, "We’re at the tipping point of the industry, which parallels the used game market in its infancy. Unfortunately, publishers ignored the used game market potential and GameStop created a billion dollar business from it. With the secondary market for digital goods, publishers still have the opportunity to embrace it and capitalize on it and PlaySpan is here to help."