Someone is spinning you a yarn - OMSP is used as the basis for dealers as well. This is from the trader manual for VRT

4.3 Used Vehicles
In the case of used vehicles, the OMSP is determined by the Revenue Commissioners at the time
of registration. In determining the OMSP, Revenue takes into account depreciation and other
market factors (i.e. age, mileage and general condition) influencing the OMSP of the used vehicle
being valued.