>> How can you enforce people not to buy or sell something, unless you decide it's a scam.

Drugs, weapons, precursors/ingredients for those things.... Certainly some of the worst and most controversial parts of modern legal codes in the west, but governments ban stuff they don't like all the time.

Where you're dealing with financial instruments, there is just so much law in place already that it seems to me it would be almost impossible to make a business using Bitcoin and not fall foul of some law somewhere.