I was well under invoice on my last vehicle purchase.  It was a built to order, so I placed a few "want ads" online through local dealerships.  First one that responded with a close to invoice I emailed back with a real low ball offer.  They responded with a decent price.  Back and forth again and a great price.
Never mentioned a trade and was never asked.
2 months later and my Jeep arrives.  While signing the paper work, I ask if they want to "buy my old Jeep".  After a good 30-45 minutes of them looking it over they come back and give me a price.  Same price that CarMax had given me the weekend before and the 14 day guarantee price nicely printed out on a piece of paper.
Do you know how easy it is to scan something, manipulate a few numbers and reprint it?
"That number is a little low", pull out folded piece of paper.  "This is what I was offered last week."  My sale to the dealer went up, a bunch!