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I just had an odd idea/strategy for if the Mint & Grand Market are both in play. I'm not sure if it's allowable by the rules. Please bear with me.
Assume for arguments sake that I have 11 coppers in play and 2 buys. (A bit contrived, but bear with me).
Once I play the treasures, I can then spend the coins as I see fit. I first purchase a Mint for 5 coins. This trashes all treasures I have in play, but the invisible 'coins' remain. I therefore still have 6 coins left to spend, but no treasures in play. Since I no longer have any coppers in play, I purchase a Grand Market for 6 coins.
Is the above permissible under the rules?
This is covered by the FAQ on Grand Market in the Prosperity rules.
Prosperity Rules wrote:
Coppers that were in play earlier in the turn but are not anymore also don't stop you; if you have 11 Coppers in play and 2 Buys, you could buy a Mint, trash all of your played Treasures, and then buy a Grand Market.