Make it a requirement of the deal. That is what I did on both of my last two DP purchases with the dealer. Either you put me in touch with the previous owner or no deal. In both cases, the dealer called the PO and got permission for me to call them. Same thing on the coach I recently traded. I left all the service records with the coach and other 'stuff' unique to the coach and told dealer it was OK to have potential buyers call me. First couple that looked at my traded rig talked to me then bought the coach.
