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Yeah, you could possibly have the issue of the backs when you've put them in your own deck, and that would be concerning
I don't know what other promos have been released, that was a LONG time ago...
So nobody is trading or selling their own unused/unloved cards?
They also sell a product called "Base Cards", which is a set of basic treasure, victory, and curse cards with updated graphics.
(I personally would just get Intrigue, it's a lot more bang for your buck unless you really like to have nicer art on your basic cards. Doubling the number of kingdom cards, with generally more interesting cards and combinations, is huge for the game's extended replayability.)
In my limited experience with 5+ player games, it's not nearly as good as playing with 4 or less, especially as the cards were balanced with 2-4 players in mind (cards like Jester and Pirate Ship get ridiculous, and you have a lot of waiting between turns. But that's just my opinion.
Not to derail, but some of my favorite games that can fit 6 people, not counting party games:
The Resistance
Malta!
Is there a strong reason you don't break up into 2 or more games when you have that many players?
Worst case scenario, you can always play Avalon without the roles - in which case it's exactly the same as the Resistance.
Is Intrigue a stand-alone? I would need two stand-alones to have two separate games.