Never noticed this myself but I suppose that's because Pigs are actually Uranium with a different name and icon. I assume you took any units that were outside your territory when you researched the appropriate tech back to your own lands? This is the Vanilla prerequisite, that units in foreign territory only get the promotion once they enter a tile you own. I think only buildings that have maintenance cost can be sold, which is a Vanilla feature. In the unmodded game, there are no buildings that have a resource requirement but no maintenance cost, so in theory there would be no reason to sell a building that doesn't have any drawbacks (like for example the Woodcutter now in AD, which has only benefits but no running costs, or any gold-producing buildings in the unmodded game). Did the adventurer have a name? Once all names are taken (which is quite likely in the last era of the game), the only thing left to do with adventurers is start a golden age, and for great writers to get the instant culture boost (which is almost always the better alternative anyway once you have a certain base culture output). This is because one specific great work of art/adventure is tied to the great person.