For ease of discussion, let's call your in-laws "Harold & Maude."
So Jay & Dona have been trying to sell this land for 4 years and they want $30-35K.
And you are asking for seller financing why - because Harold & Maude don't actually have the $30-35K?
This is what I see:
* In terms of making a pitch to Jay & Dona, i think the real leverage is that they've been trying to sell for 4 years with no takers. They MUST know that since there is no demand for building lots in this little town, that really their ONLY chance of selling the land in the foreseeable future is to sell to the neighbor who wants a bigger parcel.
*So, let's figure out how to make a deal that works for everyone. I don't know how Jay & Dona arrived at their price but if they still don't have a buyer clamoring for it, it's probably overvalued. So the first thing I would do is have a 3rd party give an appraisal of what the land is actually worth.
*I assume it will come back at a lower number than the current ask. But even it it doesn't, ask Harold & Maude if there is a lower price at which they could just buy it for cash?
*Take the appraisal (if it's less than the current asking price) and prepare a "2 Tiered offer." In a 2-tiered offer, you say, I can offer you $XXX (a lower price than they're asking and a lower price than the appraisal) for your land in CASH, it will be a simple close without most fees and without requiring bank approval. Or, I can offer you $(a higher price) if you would be willing to seller-finance, which will give you monthly income and keep you from losing a bunch in taxes from getting a lump sum.
I wouldn't complicate the conversation upfront with talk about how they can sell the note later, but certainly answer all their questions about how notes work.
Good luck!
Gail