I've had this issue before, but recently it has gotten a lot worse. Basically before this issue got worse, if I start the car sometimes nothing would happen. No cranking, all I could see were the lights on the instrument panel flash. So, then I would just try the key again and and it would start right up. Recently, it has gotten a lot worse. I would have try doing this many many times before it would start cranking.

So I thought that maybe its the ignition switch or the solenoid on the starter. So first, I checked the ignition switch by connecting a multimeter on the V+ signal going into the starter. This signal is suppose to go to +12V when you are cranking the car, sure enough I would get +12v every time I turned the ignition switch to start.

Next, I took out the starter and took it to pepboys. I replaced the oem starter two years back from them and it had a lifetime warranty. They tested it twice and it worked both times. I didn't believe them, so I went to autozone and it worked fine there too.

I explained the problem to pepboys service rep and they said that it maybe the electrical wiring or maybe the terminals on the battery have gotten corroded. Could it really be that case? Visually inspecting, both terminals do show wear, but not to the point that they are corroded. I'm not sure what to do now and I really need help to figure out the problem. What else can I do? Please help.

Batteries develop a white chaulky residue on the terminals and you get a bad connection. If you take a wire brush, sandpaper or baking soda and water and clean the battery and the cable side of these connections and then tighted the connection at the battery you have eliminated that as a cause. Sounds like starter or weak battery?
How are the headlights? Bright? What is battery voltage?

After cleaning the battery you could try carefully jumping the two terminals of the starter that are engaged by the starter solenoid. One terminal comes from the battery, the other provides power to the starter motor. By doing this you are bypassing the starter solenoid and your ignition circuits.

If it starts fine this way that that means the starter motor and battery are OK and you have a starter solenoid or ignition switch issue. If you still have issues than you have a starter or battery problem. You should check that all these connections at the starter are clean and tight just like the battery terminals.

If you are still not sure buy a new starter and install it and see what happens. I think you can return it for credit if you don't need it.

EDIT:buy the starter from Pep Boys.

Ok, so I tested this issue further, Im leaning more towards a solenoid problem, because the wire going from (+) battery to the starter, reads +12v and makes good connection; however, the wire going from the starter solenoid to the starter gear, reads 0V during cranking. Also, the ignition wire reads +12v while cranking. The only problem is i'll take it pepboys, somehow its going to magically work at the store and they will refuse to let me exchange one with a rebuilt one.

Can't you just buy a new one and return it if you don't need it?

Install some new grounds, I don't have the link...but get some cables and run some new grounds from the battery to the chassis and to the starter.

+1 you could also just replace your battery terminals. I don't think that is the issue though.

THAT ONE!