Why is all the plumbing draining slowly in my old farmhouse?
I would locate the septic tank, have it pumped out, then call a plumber with a snake and camera to check out the piping from the toilet to the septic tank. In my area a septic system is subjected to a rigorous test to see if it works properly. If not, it has to be fixed before the sale or can be sold with an "as is" sales agreement with the prospective buyer.