For a seller, this would only make sense under one basic scenario.
Seller has an old trailer that is actually worth around $1,000 to $2,000 just sitting, gathering dust and is not important to the seller's financial situation. The seller sells it to the buyer for $5,000+ and gets $1000 down. Now, the seller has minimal risk and significant upside. This is how the "tote the note" car dealers operate. Have maximum upside and minimum money on the street. Most even take it a step further and require weekly payments, so the buyer can't get too far away. But even in these transactions the seller gets a lot of personal information, verifies residence, gets personal references and the like so they can track down the seller in the likely event they start missing payments.
The seller MUST take all the steps necessary to legally transfer the title and secure a lien. NEVER hold the title until the balance is paid. If an accident should happen the seller would still be on the hook and liable for any claims. Ditto for traffic tickets etc.
Personally, it wouldn't be worth the effort and risk to make at most a few thousand dollars. I would just dispose of the $1000 rig and be done because I know that most of these deals never pay to anywhere close to maturity and most of that hoped for profit is going to be vaporware.