The Durango development kit, which is not representative of the final build, rather the final hardware specifications, was put up for sale for $10,000 on Assembler Games.
Numerous sources, including the seller himself, have confirmed its authenticity to Digital Foundry.
The kit is powered by both an Intel CPU and Nvidia GPU and packs more than 8GB of memory, although claims that it will have an 8-core CPU have been described as "difficult to believe".
The 64-bit system will be able to convert PC-based DirectX 11 games with little problem, utilising a similar PC-friendly set-up to the Xbox 360.
Recent Xbox 720 patent filings suggest Microsoft's next console will feature an improved Kinect device and two CPUs, one for gaming and another for multimedia applications.
The alleged filings support some of the details discovered within a set of alleged leaked Xbox 720 documents from 2010.
The 56-page set was removed from the web following an intervention by Microsoft's legal team.
It claimed that the Xbox 720 will feature Kinect 2.0, 3D technology and the long-rumoured Xbox TV.