Tumble Weed,
I think I saw a similar thread a few days ago.
True, they may make money on your trade in, but what you owe on the unit is not the dealer's problem, because that is between you and the financial institution that has the paper on your current fiver. The dealer needs a clear title so he can resell your trade.
For example, if you owe 10,000 dollars on your rig, but the dealer is only willing to give you 8,000 dollars as the trade in, then you have to come up with the other two thousand to pay off your trade so the dealer has a clear title.
Typically, during sales of this type, the dealer will subtract the stated trade value from the agreed to price of the new trailer to get the final sale price of the rig, but since the dealer had to pay the extra two thousand to get the clear title, this amount will normally just be added to the new contract (or you can pay it in cash to keep it out of the new contract). Remember that you are not required to accept the offered trade in value and can shop around for a better deal.
The dealer has no liability or control over what you owe on your current rig. Other customers may owe nothing or more, but that is the owner's concern.