Back in the day, Nintendo issued demo units to retailers which allowed them to show off multiple NES games on a single system. The Nintendo M6 is one such example.
The M6 is comprised of a central console which would have several game ROM chips inside, and the user can switch between them at will. Two controllers and a NES Zapper are also featured, allowing customers to sample a good range of the console's capabilities in-store before making a purchasing decision (or, as was more often the case, giving kids the chance to get some free gaming time in while their parents were shopping elsewhere).
Needless to say, as this wasn't a system that was sold to the general public, Nintendo M6 units are incredibly rare – and one has just gone up for sale on eBay.
The unit appears to be in very good shape, and the seller – the American Cancer Society – says it has been tested and is in good working order.
The auction has already passed $2,000 USD, and has over nine days remaining – so we can safely expect this one to reach a decent price.