On the service I would agree it sounds illogical. First of all I would ask the sellers the question. Below the service this move could be prompted by a number of issues from tax consequences to personal issue.
One other point: If they've been trying to sell it for 4 years, it's overpriced. Period. Yes, maybe it doesn't show all that well with you there. But 4 years? And that should give you at least a clue on what a reasonable purchase price should be.
My suggestion: Get a Realtor to do a CMA to find out what the house is really worth. Then work up some numbers for the sellers, showing the relative cost of keeping the house empty, no cash flow, for say 6 months (or longer), plus the real estate commission upon sale, versus your proposal of a lease to own, constant cash flow, and a purchase, albeit probably at a lower amount than their likely overinflated price.
Hope that helps.