I am interested in only certified used vehicles. How does dealer brand X certify brand y?

they cannot- that protects GM dealers- Ford is the same way, as is Honda, etc.- if you don't sell them new you can't certify them- you want a mechanic certified in that brand to do the work. It has nothing to do with not being willing. You can purchase a used car warranty and have similar coverage, and if their price is good on the HHR, that may be the best option, becasue certified typically cost around 1000 more for a similar car. I hope this helps.