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If you need a second hand mountain bike, Bike Himalayas usually has some well maintained ex-rental bikes available which are perfect for the streets of Kathmandu.
Trek 3900, 2009 and 2008 models, moss green colour with different frame sizes 16,18,19.5 and 21 inch. Price Starting from 15,000 to 25,000.
Contact Ranjan of Nepal Mountain Bike Tours firstname.lastname@example.org and http://bikehimalayas.com/contact.php