Is there any reliable way to measure the outcome to see if it worked?

If we assume that the appraisals are disconnected from the winning bids*, then couldn't one just see whether the ratio of sale:appraisal is increasing? If the appraisals are honest, then any jiggery-pokery should alter the ratio - eg. a successful manipulation will lead to people paying an average 93%, where they used to pay 90%.

• that is, there is no feedback - the appraisers don't look at recent sales and say, oh, I've been lowballing all my estimates! I'd better start raising them.