Depends on whether it is a MK1 or MK2 model.
The first ATCOs were American imports from the late 1970s to 1986 and included several model numbers including 11/36 and 11/36E.
The later ATCOs are the dark green ones with a black front, made in the UK from 1986 until 1992. Again there was an 11/36 model.
If it’s the later model then any good value is dependant on both the cosmetic condition and the working condition of the machine. Age can make the dark green paintwork, plastic trim, seat and stitched steering wheel look very tatty.