A loan amortized over a huge period of time isn't the same as interest-only. Both are methods of reducing the monthly payment, but in different ways. Interest-only means exactly what it sounds like - you make only interest payments for a certain amount of time (generally 10 years on a typical IO mortgage), and then you pay principal and interest for the remainder of the loan (usually there is no balloon). A longer amortization period means you pay principal and interest every month, but over a longer period, so the monthly payments are lower. There may or may not be balloon period at some point (such as the 30-year point).
I don't know if there are rules on private lenders, but if this seller won't budge on price or interest rate, I can't see them being willing to amortize the loan over 50+ years. If they are willing, maybe check with a real estate attorney on whether there are any regulations against it. I don't think there should be in a private financing situation, but I don't specialize in this field.
One thing to be careful about - most lenders are going to charge you a higher interest rate for an interest-only loan or a longer amortization loan. Depending on how much higher the rate is, you might not even be saving much on your monthly payment. Just be sure to figure out exactly what you're saving and if it's worth it.
I agree with @Stephanie Medellin that mortgage lenders used to have 40-year loans, but those have been gone since around 2007.