The rate is usually determined by your risk level. When the seller holds paper, they don't typically underwrite in the same mechanical way that a bank would underwrite (with complex software evaluations, etc.)
That being said, 25% interest means nothing if you can't deliver payments. You don't want to come off as too needy, or willing to accept unrealistic terms for yourself just to get the deal through. Sellers can smell warning signs like that.
Is there any way you can put cash down to lower the LTV?
"The first lesson of economics is scarcity: there is never enough of anything to fully satisfy all those who want it. The first lesson of politics is to disregard the first lesson of economics." Thomas Sowell