The Tracker bike was used by the Princess of Wales when she lived in Althorp, Northants, in the 1970s.
The bike was sold at Keys auctioneers in Aylsham, Norfolk, to Alan Goldsmith, of The House On The Hill Toy Museum in Stansted, Essex.
"We decided it should be saved for the nation and also kept in East Anglia," said Mr Goldsmith.
"I would have been prepared to go up to £10,000 because I believe Diana was the biggest celebrity icon of the last century."
Keys said the bike had originally been bought for £20 by Maudie Pendry, a housekeeper employed by Diana's family.
When the housekeeper died her family decided to sell the bike, it said.
The lot, which sold for a total of £1,350 including the buyer's premium, also included a letter written in 1982 and a Christmas card from 1994, both signed by Diana, who died 10 years ago.
Source: BBC- UK News May 18, 2007