Pistol:
Some thoughts...Could be wrong.
Is it possible to pay off the lien, yourself, even if you have to use another line of credit (like a credit card?). Makes things much smoother.
Otherwise, contact your lien-holder, and see how they recommend handling things. The buyer will want (or *should* want) to pay off the balance of the loan directly to the lien-holder. Say if you owe $20,000 and sell for $25,000 you should get one certified check for $5000 and the bank should get $20K.
There's some paperwork involved. It's handy if you can get all parties involved sitting down at the bank.
As Mike points out -- don't make his mistake and let the trailer leave with your plates. Some states don't even require them, on the trailer, and regardless -- that burden should be on the buyer. Have fun!