The sentences with incorrect subject-verb agreement are:
- "Either the serving platter or the bowl are in the bottom cabinet next to the dishwasher." (It should be "Either the serving platter or the bowl is in the bottom cabinet next to the dishwasher.")
- "The people who attended the fair was there because they were interested in purchasing artwork." (It should be "The people who attended the fair were there because they were interested in purchasing artwork.")