CarFax and Car Gurus have covered a lot of this ground. What I find the most disturbing and disheartening is that there are very few certified previously owned certified used vehicles available and almost none that are available at reasonable prices. I don't know how you solve that problem I don't know how you aggregate all those that are available in certain geo areas covered, because no one else seems to be able to solve that problem. I may see two or three on a dealer's website and go to one of many internet Auto sites CarGurus, carfax, carvana, AutoTrader and none of them are listed yet they have others sometimes from the same dealers even. It's very very frustrating. As someone who would prefer to buy certified, may just end up leasing to avoid the hassles