i put a space after www. so links show up.
www. tractorhouse.com (tons of used tractors)
www. machinetrader.com (tons of construction tractors / vehicles)
www. ironsearch.com (no were as big as above 2, but lists some extras)
www. craigslist.org (not the best, but you still see listings)
www. searchtempest.com (allows you to search multi cities at once of criagslist.org)
www. ebay.com (might as well, the list tractors as well)
there is a "user" trader/seller forum here on TBN as well. clear down at the very bottom of page. www. tractorbynet.com/forums/
check local trading / advertising papers. i would imagine most local papers have an online website to see ads for tractors.
local auctions, can be a good place to find tractors as well. normally there are "large machinery" auctions 3 to 4 times a year within 1 to 2 hour drive i would assume from most folks locations.
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if you buy a used tractor, you best bet, is to make sure it comes with a FEL already attached and comes with it. trying to find a used FEL at a cheaper price and will fit on your tractor without a lot of modifications or extra costs for brackets can be very expensive. not to mention just buying a new FEL.
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the higher your price range say 10,000 bucks, the larger search radius i would be willing to look, say up to 3 to 4 hour drive. and i would imagine you could find something within a month that would be a good deal.