1 -- Selling the cocaine one finds would invite questions from those who lost it. Probably not something that could end well. If there is a bag of cocaine lost, someone is actively searching for it.
2 -- Selling it puts you at risk of getting sent to prison. Again with the not ending well.
3 -- If you do manage to sell it then you are a drug dealer to the person you sold it to and will carry that reputation from then on. Plus, users who get caught will give you up. See #2
4 -- Giving it away is really the same as #3. But lets be serious, this is really not remotely an option for anyone in reality. You dont give away a bag of cocaine. Pot, I can see, but not cocaine.
5 -- If you are a cocaine user, then your gonna sell some of it and keep some of it. A bag of cocaine is too much for one person. Cocaine is a social drug. Therefore someone is gonna ask you for some and your gonna end up selling some of it to them. Thus, see #1,#2, and #3.
So to a logical non-nefarious mind there are really only two options. Flush it or give it to the cops. I chose to give it to the cops because then there is no way the guys who lost it might think I used or sold their product. Just a normal citizens turning in contraband.
Of course, I may have just watched too many movies.