"Apple does NOT let these phones out of their possession yet I was accidentally sent one upon renewing my contract," a description posted on eBay by the seller said.

"This is the real deal." Bidding for the silver iPhone 6 prototype with 64 gigabytes of memory had reached $99,800 with 198 bids logged by late Monday morning in California, where Apple is headquartered.

The seller, identified in online reports as a US man, described the iPhone as running in "true developer mode" on testing software called "SwitchBoard" and not on Apple's latest iOS 8 operating system.

Bidders are warned that there is no guarantee the iPhone will make calls or that the camera will function. Shipping will be free given the dizzying rise in bidding, which is slated to end on October 9.

The device was plugged into an iTunes account and recognized as an iPhone, the description said.

The winning bidder is promised the device, box, charger and headphones, along with the device's paperwork.

"I would like to reiterate that this phone is being sold in As-Is condition," the seller stressed.

"The only reason you may return this device is if the device arrives DOA," or dead on arrival.