Sagging Floor Joist in House Built in 1925
It's not unusual to see sagging floor joists on houses that are nearly 100 years old. Sagging joists can be adjusted with jacks, then permanently supported with a wall in the basement (or knee wall in a crawl space). This sort of thing is often best done by professionals though, because if done on your own too quickly you can cause other damage, or if done without permits, it can become a liability to a future sale of the property.

Black plastic may have been added as a vapor barrier over under-floor insulation, the 1" holes may have been part of a radon remediation effort. In other words don't jump to conclusions about fraud.