A motocross bike owner from Brighton put his machine up for sale on the ebay auction website then lost it to a thief.
The bike – a Yamaha ZY125 – was stolen in a burglary along with two Trek 4500 mountain bikes.
Sussex Police said that the burglar struck at a private garage in Saltdean between 7.45pm on Friday 30 September and 6am on Saturday 1 October.
The blue and white motocross bike, which has distinctive large black number nine stickers on the sides and front, is not registered for road use.
The mountain bikes are black and white and fitted with disc brakes.
The Yamaha was offered for sale on ebay by the victim and detectives are investigating a line of inquiry linked to this.
PC Jon Attfield said: “We urge caution to anyone selling items online or through classified ads when giving out address details to potential buyers.
“Where possible obtain full names, contact details and a landline telephone number for buyers prior to meeting.”
Anyone who saw the burglary or who has seen the bikes or has been offered a chance to buy them is urged to call Sussex Police on 0845 6070999.
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