I have experienced it before selling my truck and it turned out to be a scam. They called and said the represent buyers and want to be paid up front for the sale. Their logic behind it was they were sending a buyer to look at my truck who was going to take it, the money down was to make sure I did not change my mind, if I did the buyer received the 500 for wasting their time. When I searched it on the Internet found thousands of complaints which helped me save $500 lol.
One item I have seen and not a full scam, many dealers will sell your rig on consignment. 5% of the sale seems average and you need to give them a min they can sell it for. As well if they can sell it for more then you want they have a bonus built in. For the most part it is fair but I have seen dealers use these as examples on why it is better to buy new or to upsell.