i'm confused, "sua" can be used for both "your" and "his/hers" right ?
Sua = your, his, her, its, their.
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".
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"