1988 Nissan Sentra Park it for the night then it wont start

Engine Mechanical problem
1988 Nissan Sentra 4 cyl Automatic

I just traded a good truck for what I thought was a good car I met the guy out in town test drove the hell out of it and now it wont start right the guy said it had a new distributor, cap, and all kinds of other things. So it's been parked for about a wk. & 1/2. We found out the O2 sensor was bad replaced it, the battery, timing belt, put a half tank of gas in it, my boyfriend and his friend knows somewhat alot about cars. But anyway it would'nt start early in the morning yesterday morning but around 5 pm it started right up and we drove it around for I think about 4 &1/2 hourse and we did turn it off a couple times. We got pariniod and cranked it up fine every couple hours until 2:30 am it wouldnt crank again. Got up at 9 am it wouldnt carnk. What I noticed about it yesterday was every time it wouldnt crank it didnt make a certain noise but when it didit made this noise I think it was like the fuel pump getting ready. Like it was moving. But now its not making the noise and wont crank.I've tried the 1 sec. Off & on and it worked after we got it cranked the 1st time.I think it wont crank after it gets cold at night.I need Help!