It may simply be a case of a new salesman wanting to build a customer base, making next to nothing on the deal.
Or, he has a customer that refuses to buy new for whatever reason, and wants a "good used 2520". Some folks have the mind set that they always lose when buying new. That would account for the replacement of old implements with new. Sometimes old gear IS better than new, in terms of material quality and the amount thereof.
If he's selling the used 2520 to a cash customer, 0% financing makes little difference in today's world of poor interest rates on investments.
I looked at used tractors for a couple of years before buying new, it soon became clear to me that used wasn't much of a savings, unless I wanted to buy 10 year old or older machinery. Three year old tractors carried a sticker price of less than $3000 difference from new.
There are often two distinct types of customer, those that buy only new, and those that buy only used. I think the salesman has one particular customer that's willing to pay close to full new price for your tractor, refuses to buy elsewhere, and is more comfortable buying from a dealer. More "money than brains" comes to mind, but whatever makes 'em happy.
If that particular dealership has a 2520 they'd like to move, and are willing to cut their profit to the bone to do it, they increase their customer base at the same time. It's a calculated risk, long-term return approach to business, maybe aimed at getting into a market they feel will serve them.
I doubt JD would do that across the board, no sane manufacturer would put themselves in that position unless it showed a larger "+" on the bottom line.
Sean