if a load bearing wall was removed, use the inspection clause to RUN from the house. DO NOT trust a contractor used by the seller, it could be his cousin who is trying to help him unload on an unwary buyer. the only way to fix is a major job of installing a steel beam to support the upstairs after it is jacked up. this will require a period of time as the jacks are increased in lift every couple of days to allow the house to adjust to the changes.
a long and expensive fix. request that the present owner install the beam prior to closing, or allocate necessary funds to repair to be held from purchase price at closing. IMO the best recourse is to look for another house without structural problems