Same topic came up on another forum I frequent. I looked at the CRS and the emissions statute states that any vehicle sold that is "roadworthy" must have an emissions test. So I would think as long as you state that the vehicle is being sold as not "roadworthy" you would be ok.

(15) "Roadworthy" means a condition in which a motor vehicle has sufficient power and is fit to operate on the roads and highways of this state after visual inspection by appropriate law enforcement authorities. In order to be roadworthy, such vehicle, in accord with its design and use, shall have all major parts and systems permanently attached and functioning and shall not be repaired in such a manner as to make the vehicle unsafe. For purposes of this subsection (15), "major parts and systems" shall include, but not be limited to, the body of a motor vehicle with related component parts, engine, transmission, tires, wheels, seats, exhaust, brakes, and all other equipment required by Colorado law for the particular vehicle.