We are talking about commodities - things that are bought and sold, including art works and antiquities. Basic supply and demand. How could the black market be made less black? Well, obviously law enforcement plays an important role, as do international agreements  that protect archaeological sites, but my primary point is that if people didn't turn a blind eye to the origins of what they are buying, the problem would be lessened. You seem to think this is a trivial issue, but it's not. Sites have been destroyed, museums depleted (look at what happened in Baghdad in 2003, after Rumsfeld declined to station troops at various cultural sites). Museums and collectors are still returning stolen art works to Jewish families.