The gears won't shift from first.

I'm interested in buying a used 1993 Dodge Spirit V6 but I smelled oil when test driving it and it won't shift gears also. The seller told me the car has been sitting in the garage for four months and has not been used. I'm wondering if the transmission screen just needs to be cleaned. The seller also said that it just needed a tune-up and brakes. I don't know how many miles is on the car or what work has already gone into the engine.