Having issues with Iphone 4?

I recently bought a Iphone 4 at&t unlocked 16gb for 50$ on craigslist(not a bad buy… I'm currently on Tmobile network with this phone)
The guy selling it to me didn't mention any problems with the phone but for 50 bucks i honestly didn't care what the problem was.

The first 3 weeks the phone worked wonderful not one sigle problem, then i began to have issues with it. I thought it might of been a fake but i looked it up and if its a fake its the most realistic iphone there's in the world.

so here are he problems:

1. When talking on the cellphone i have to put it on speaker or i can't hear anything
2. The camera stays frozen when i try to switch it to the front camera
3.can't send any messages to any android user(only people with imessage)
4. The headphone jack is not working.
5. And down volume isn't working as well.

i was planning on getting it fix next week but i'm looking at a hefty fine.i've already reset it and tried anything people have suggested on other sites but nothing seems to work. I don't get it i haven't dropped it or anything and everything was working absolutely perfect so what could it be? Water damage from previous owner? Maybe he dropped it?
how to fix?
Any suggestions…

-.- no negativity please

When iphone's are dropped THEY ARE RUINED period because of any little chip or anything inside the phone, 50 dollars phone i. Can't say you gor ripped off bur usually if someone sells sumthing "good" for cheap it basically he had the same problem you are having with it, you better off getting a new phone