I Think My Boyfriend Is Using Cocaine. How Can I Tell, He Disappears, And Is Selling Some Of His Things. Then He Gets Angry And Then Sad And Starts Crying? I Am Confused.

Well those symptoms are suspicious, and do fit the description of a user, but he could just be having some money troubles.
I mean, obviously he is either way, but drugs may not be involved and he may just be stressed out.
Your best bet is to catch him when you're sure he's sober (after several hours of being with him) and relatively emotionally stable and calmly ask him whats going on. Don't make any accusations, or assumptions, that may just anger him and cause more problems. Just make it clear you're worried and you're there for him, he may just open up to you about it and let you in.

Other than that all I have to say is watch out for yourself, be sure you're in the relationship for you and your happiness, not his. I'm not saying for you not to be there for him or not to care about him, just don't subject your happiness for his poor choices and way of life.