i'm confused, "sua" can be used for both "your" and "his/hers" right ?
Sua = your, his, her, its, their.
Why da instead of de?
Both of them are acceptable.
I suppose if de + o (article) = 'do', then de + a = 'da'. With possesives you can say the article or not, am I right?
yep, this is the why you can say de or da (de+a), not do (de+o) because camera is a feminine word just take a look: http://duolingo.com/#/comment/64947
Why is her or his camera not correct?
That is correct too. But, sincerely, its more likely you to hear "eu gosto da câmera dele/dela" instead.
Why is "I like his camera" wrong? I am confused. "Sua" means either "your" or "his/her"
Câmara or câmera? I've seen both. Which one is more common?
"Câmera" is used in PTBR, "Câmara" is used in PTPT to mean "camera".