Bills of Sale, no matter now craftily written aren't the way to go and wouldn't matter in the case of lien. I understand what you're doing on buying older stuff, with little likelihood of lien, however. But newer stuff?
Simple reason?
The "seller" doesn't own it. Not really. The lien holder "owns" it or at least proportional rights to it, in that money is still owed. I don't have the right to sell something unless I've cleared another's equal claim to it. Just as if I had two names on a car title or owned a business with an equal partner.
Thus, a bill of sale isn't where I put my trust. When I buy used, and I often do, I want the title cleared, the lien cleared, and the UCC cleared, whichever applied. Otherwise, the BOS is meaningless, at least to me. First things first.