The only thing that would scare me about that is until it is paid off it is owned by the finance company. I am not a lawyer and may be wrong but I believe if he doesn't pay it off and misses payments and the finance company wants to repo it you won't have a legal leg to stand on other than suing the seller. To be on the safe side you may want to pay off the lien and give the seller the difference (if there is any).
Two examples:
www.greentractortalk.com
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www.farmcrediteast.com
When buying untitled used equipment, ALWAYS obtain a Bill of Sale and a photocopy of the sellers I.D. The Bill of Sale should STIPULATE “Sold by John H. Doe who guarantees to perfect this sale at his own expense, in the state of (buyer’s residence) with clear titles, and free of all liens, claims and encumbrances”. I know of several instances where used equipment was taken in for service to find the service facility had an unpaid invoice or mechanic’s lien from previous owners and refused to release the equipment back to it’s present owner.
I am not a lawyer, but I insist on language such as the above whenever I buy anything used. (And when I sell something I also include the words “Sold As-Is, Where-Is, without warranty as to fitness of purpose or merchantability”.