i think you should complain to the RTA.
Not even the tax system is that bad.
When calculating capital gains tax, so long as you are dealing at arm's length (ie. genuine sale, not just to a mate etc) your capital proceeds will be what you actually receive upon sale. Similar concept here - if the auction house (or private seller for that matter) genuinely sells you a car for a cheap price, your stamp duty should be based on that IMO.