"Selling free of liens.."
It's always a good idea to require the seller of anything (used goods in particular) you purchase to warrant that it has and will transfer good title. However, "free of liens" is not, by itself, sufficient. Liens are a specific type of legal interest that can attach to goods. For example, if you send your car/boat to be repaired, a lien can be attached/claimed by the repairer.
The all embracing "term of art" which one should seek to incorporate in a contract for sale and purchase of second hand goods or a bill of sale is that the seller sells "with full title guarantee". This term incorporates, by statute, implied covenants which cover liens, charges, security interests and other "encumbrances".
Note also that if you are purchasing from a seller who is acting in the course of a business, you have staututory protection under the Sale of Goods Act which includes an implied term that the seller has and is entitled to transfer title to the goods - and the seller cannot exclude that term regardless of any purported attempt to do so (Unfair Contract Terms Act).
However it is possible, even where the seller is a finance company, that title may be transferred subject to a security interest of which the seller is unaware. Therefore it is probably a good idea to ask for "full title guarantee" (but seller may be reluctant to grant this).
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