I can get a 2001 V6 camaro convertable with just over 120K miles for $3500
the car is in "good" shape with a nice interior and top.

BUT even though it seems to run good, it will not pass emissions. And now has a overheating problem.

The owner has replaced the radiator, and thermostate but the car still overheats.

He now swears it must be the waterpump ($50)

But I am very sceptical because the pump does not leak, every failed waterpump I have ever seen leeked out the weep oil.

I am thinking maybe the fan is not coming on ?
or maybe a bad sensor somewhere ?

Is there something that generally goes bad with these engines that casues this overheating ??????

NEED HELP ASAP as I think the car will sell FAST at this price

Is your SES light coming on. I think usually in these engines if the fan doest come on when its suppose to or there is a bad sensor the computer should though a code. If so just borrow a OBDII scanner from Autozone and scan the PCM.