I purchased mine used from DARCARS in Rockville (previously Courtesy). Salesman was Travis and he is great. I haven't had any experience with service yet for my XF but they have been excellent with my Sebring Convertible and Pacifica.
During my XF search, I found that the dealers were ok with a transfer between the same dealers i.e. all the DARCARS will transfer with other DARCARS. I don't think you will get the "best deal" with a transfer between DARCARS and let's say PASSPORT (if they will do it at all). That being said, PASSPORT down Rt. 395 in VA seems to carry the largest selection. My experience was that once they came down to a certain price that was it, even when I walked away. They called me the next day and offered another XF for the same price as the one I was looking at (different color) but wouldn't budge any more on the price.