Only bought a property once on a land contract. It was 5 years with a balloon payment at the end. Worked out well for both parties.
Concern from sellers stand point with LC is how what collateral will they secure the 40%? Also, what happens if they default and have damaged buildings, had the land timbered, etc., how do you recover? What if they don't pay and you are chasing payments?
Why are they not financing 100%?
Personally I would not sell under a LC. I'd talk to an attorney to see how to construct contract and what are the risks for you the seller. Big issue is how do you foreclose on LC if they default. Doing anything with the courts is a slow & expensive process.