88 7mgte cranks but wont start?

i was checking a supra down the street from me, the guy said that the motor have been rebuilt, but i cant get it to start. everytime i turn the key the engine would crank but would not turn over, it wont even hesistate a little , just wont turn over. the car looks real nice and i would like to buy it. any advice would help. thanks.

then i found out that polaritys of the battery was backwards

can this damage the car?

I have heard of plenty of electrical problems from putting the cables on backwards with all sorts of cars. Start with the fuses in the engine bay...then those in the kick panel.
Good Luck

thank u