But even if it is known that a car will require $1500 worth of extra work then a used car manger would be a fool not to pay $1500 less than one that did not need that work. In such a case, the dealership is not "paying $1500 less than its worth", the dealership is paying just what it is worth.
Exactly. That being said, they will always try to get the best deal.
When I test drove the Sportage, the salesman and I talked trade-in values. I told him I knew the black book value range (14,000 to 17,500) for my car and that at least one retail evaluation of my car put the sale price well over 18,000. He advised me to sell it privately, stating that the used car guy on the lot probably wouldn't give me over 15,000. It might even be around 13,000. That's a potential difference of 5,000.

Obviously, estimates eventually fall by the wayside in the face of an actual sale but they do give a framework in which to bargain. Being a bit of a lazy sod, I might still consider a trade-in when the time comes but the price difference is an eye-opener.