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You could ring eBay and tell them, give them the serial number and any other identifying detail, that way they also will be keeping an eye out, every time I have had cause to give them a ring I have found their operatives very friendly and most helpful. Ebay own Gumtree as well, you could also put it out on all the other social media platforms as well, Facebook, Twitter, Whatsap etc. The wider coverage you can get the better, It's been known that thieves have seen this sort of widespread coverage and have actually been shamed into returning stuff, 2 numbers to try, 0345 3353229 or 0800 3583229