eBay bidders have offered more than $30,000 for a World War II-era German Enigma cipher machine, a sum that's "on the high side," according to a dealer who has sold other models.

The online auction, which is scheduled to close Monday, is for a three-rotor Enigma machine manufactured in 1941, said the eBay listing. The unit, however, is missing a serial number, and the rotors -- which typically are stamped with the same serial number as the Enigma itself -- appear blank.

"Serial numbers has [sic] been removed," the listing states.

The Enigma, which is being sold by a Munich-based company called Sales Service, has already been bid higher than most recent at-auction or private sales prices, according to a listing of recent Enigma sales.

"It's in the ballpark, but to see [a three-rotor] Enigma at $30,000 would be unusual," said Ron Watson, a collectibles dealer from Manitou, Manitoba, Canada. Four-rotor Enigmas, which were used primarily by the German navy, are much scarcer, he said, and so command premium prices.

Watson said he recently sold a three-rotor Enigma in perfect working order for about $24,000.

"You can't expect top dollar unless [an Enigma] is complete and original," he said. "If it has replacement parts, they're not the original parts." He said it was likely that the rotors had been rebuilt or replaced, and were sans serial numbers for that reason. "That's okay, as long as it's upfront."

The Enigma holds an attraction not only to collectors of World War II memorabilia, but also to those fascinated with computers and their history.

"The Enigma is why we have computers today," said Watson. "The computer had to be invented to work out the breaking of the cryptology that's used by the Enigma. So the computer you're typing at now actually goes right back to the Enigma machine."

Current computers and the Enigma recently crossed paths, when in March a distributed computing effort dubbed the M4 Message Breaking Project used several thousand PCs to crack two out of three messages intercepted in 1943, but never decrypted by the vaunted British research facility at Bletchley Park.

"Breaking the [Enigma] code gave the Allies the advantage of knowing what the enemy was proposing and planning," said Watson. "There's no more important 'spook' item for a museum or private collector to own."