First, Tesla is selling the loaners in a back door way to act as a stocking dealer. Some people just do not want to wait. New loaners are untitled and thus get the $7500 credit.
Trade ins are valued at full invoice (not including local taxes) minus 17% minus 1% per month (and for some) minus $1 per mile. Yes, some people have gotten say a P85+ loaner while their P85 was being serviced and decided to stroke a check and keep the loaner. I've also heard of another 60 owner that got a P85 loaner and kept it so, yes, Tesla has access to generating CPO cars. I have no idea if they have finalized the CPO program sufficiently to move those cars. When they do, the $7500 rebate will be accounted for in a price discount.
I did as another poster suggested and simply sold mine retail. The only consideration with this in a state like Florida is that we get a sales tax credit for the value of a trade in. A secondary market sale does not go through Tesla thus you do not get the sales tax credit. In the case of Florida, you must clear at least 6% more with a retail sale than Tesla is offering.
What is meant by CPO?