Great Works sales

So, you can swap them. I assume if you capture a city that is housing one or more Great Works, they get transferred to the new city-owner. (If not, why not?) Why can't you sell or buy them like any other limited resource?

GW sales would be too easily abused by the human player, turning culture victory into another "amass and spend enough gold" victory.

I do think that great works or artifacts should be negotiable, only if done right. Outright buying or selling might indeed be too imbalance. I suggest GWs and artifacts to be negotiable only as peace deal. Also moving out GWs or artifacts or swapping them with another civ should produce a civilian unit akin to trade route which can get plundered. Currently, you can move out GWs safely if the city is about to get conquered, while the AI does nothing about it.
Also while the Beta fall patch gives benefits for using archaeologist in other civ or city-state's territory, I find it befuddling that if you run out of artifact slot, you're forced to create landmark, even though it's in a middle of an island where no one occupies. Why not make extra artifacts in storage and available to swap for trade?