Why aren't passengers trained and evaluated for operating emergency exits in commercial aircraft? The passengers sitting on the overwing exit doors are expected to operate the doors in times of emergency, with only a verbal instruction. Is that enough?
Q. Why can't we have a designated area in the airport where a setup of emergency doors are kept and the passengers are trained and evaluated there before being alloted the seats on the emergency row. It can be short 2 min course, where passenger actually opens the door and puts it aside.

There have been instances which highlight that proper training is required for passengers Soource:

Passengers will frequently open an exit as soon as evacuation begins which may result in them evacuating into danger. This occurred in the Ryanair engine fire in Stansted Airport in 2002, where passengers evacuated themselves onto a burning wing, despite airport fire services personnel shouting at them to return inside the aircraft and evacuate via a usable exit. Source

Another hazard in the use of overwing exits is them being improperly opened (usually a result of passengers in these seats not paying attention to the verbal briefing provided pre-departure, or not observing the opening instructions on the safety card and on the exit). The majority of overwing exits involve the passenger physically removing the hatch from its frame and disposing of it outside on the wing without blocking the exit. Research conducted at the Cranfield Institute in the UK Source


Above situations taken from Wikipedia
https://en.wikipedia.org/wiki/Overwing_exits#Hazards
Former TWA here. 

Emergency exit seats used to only be given to passengers who are ready, willing and able to assist in case of an evacuation. They could not be assigned in advance, the passenger had to be asked and evaluated in person by the boarding agents, who would have been held responsible if a not-able passenger was to be seated in any of those rows/seats. This used to be the rule, it was deemed that safety was more important than selling seats. 

Maybe the rules have changed since?