I'm glad I'm not the only one who was confused by this question! (Although, if we get the really bad weather which the tabloids keep telling us we're going to get this winter, a tractor attached to a car might be a good idea!).
I'll try to provide a serious answer (on the assumption that it's actually a 'tracker' which is referred to:
Theoretically, there's no reason why the unit can't be removed and sold on. But you won't be able to interest anyone in purchasing it unless you can also provide them with the relevant information regarding registering the unit with the tracking company and (if it's the type of unit which automatically switches on when the vehicle is stolen) the annual subscription details. If you're not sure of this information, the first thing you'll need to do is to establish which company the unit is registered with and then contact them for details about transferring the unit.
The phone numbers for the main service providers are listed at the bottom of this page:
http://www.autoexpress.co.uk/product_test/47471/personal _devices.html
Chris