[HelpPlease]

I need some help. I bought a used blackberry curve 8900 on ebay (it was listed as unlocked but wasn't I had to unlock it).A week later something weird happened. I had my blackberry curve 8900 fully charged and I went to bed, the next day when I woke up the blackberry wouldn't turn on. I tried turning it on then plugged in the charger, the red light came on then a battery sign with an X on it. Well I asked about it at the phone store they said I needed a new battery. I bought a new battery placed it in and it was working perfectly but the battery was dead so I started to charge it again with the wall charger. Then I noticed the light had stopped blinking. I tried turning it on and it loaded up with white screen then blanked out. I took it to the phone store they tried their battery and it came on but the screen was dark. I couldn't leave it there so I took it with me and tried USB charge and it still wouldn't turn on. The next day I gave my cousin to take to the phone store. My cousin came back said he didn't take it to the phone store because the phone came on and it was working perfectly. I tried it out and it was indeed working perfectly. Then the trackball froze up and I whenever I removed the battery the trackball would start working again. Then now, it won't turn on again, I really don't understand this phone, I tried everything, what could possibly be wrong??? Please Help!