Fixing a sagging foundation
A 1" to 16" slope is huge.  that would mean a 10 " drop in apx 13 feet.  Did you mean 1" in 16 feet?  Regardless, the fact that you are asking the question indicates that you need a professional to take a look.  There are many possible reasons from maybe the cut center beam, collapsing or crushing support posts(top or bottom), settled footings, etc.  This is a common situation in very old houses, especially ones built on stone foundations. A simple answer would be to jack and support the offending beam(s) to restore a level floor, *however*several other factors need to be considered before jacking can be done. Factors like chimney/roof coupling, door and window reactions, tile floors, millwork/trim, plumbing connections........the list goes on. Find a competent general contractor that has experience  working on older homes. That person should be able to determine the root cause, then formulate a plan to fix the problem. To buy the house or not might be determined by the repair costs and advise this person gives you. I would also have a complete home inspection done before even considering making an offer to buy this house.