Shop.com, a Monterey, California-based comparison shopping service backed by Bill Gates and Amazon.com, has been sold to Market America in a deal of undisclosed size. The combined company will have more than 650 employees.

Gates' involvement with Shop.com goes way back, with the billionaire listed as one of the company's founding investors in 1997. It is unclear exactly when Amazon invested in the company. Other investors include Yahoo and Oak Investment Partners.

Interestingly, Market America -- which describes itself as an Internet marketing and product brokerage company -- developed an offering called MA Cashback in which it offers rebates to people who shop through the service. You may recall that Microsoft pulled the plug on a similar service, Bing Cashback, earlier this year.

It is also interesting to note that Amazon.com -- which has been highly acquisitive in recent months -- didn't purchase Shop.com. (At least as a defensive move for the URL). Now, Market America appears to have its sights on becoming a more dominant online retailer and hints at taking on Amazon.

"Our business model rewards customers at every stage – by making their shopping easier and more efficient, and rewarding them with cash back for shopping with us," said Market America CEO James Ridinger, in a press release. " This unique strategy positions us for explosive growth and to compete head-to-head with the biggest, most dominant shopping sites on the Internet."

Here's a video demo of how Market America works, including how the cash back system works. The system has a bit of an Amway element to it as you'll see below.