In reply to Pursued by a bear:
Probably a algorithmic pricing fail.
Seller One sets pricing algorithm to be update pricing every morning to be slightly higher than Seller Two (say 1.10 X Seller 2 Price)
Seller Two sets algorithm to update price every evening to be slightly lower than Seller 1 (say 0.95 X Seller 1 price)
So if both start their prices at £1, after the first morning SellerOne has updated the price to £1.10, then in the first evening SellerTwo has updated to a price of £1.05,
At the end of the second day SellerOne will be at £1.15, SellerTwo will be at £1.09.
If that continues in 30 days they'll be at £4.12 and £3.91.
In 300 days they'll be at £571693 and £543109
https://www.wired.com/2011/04/amazon-flies-24-million/
Post edited at 11:38