I feel like I need to speak strongly here, sorry if I come across as antagonistic, but my point doesn't appear to be getting across. I really don't understand the reasoning behind thinking RM is better than Copper in most decks and explaining my reasoning doesn't appear to be getting through. I feel like this is not close, not in the slightest.
Ruined Market is much, much worse than Copper. Given you're going to draw it anyways, having the option of exchanging an Action for a Buy is really, really bad compared to just having another dollar to spend. For every edge case you can come up with where RM is "better" I can come up with one for Copper just like it (Stables, Bank, Coppersmith, Moneylender, Apothecary. I would bet money that I can make this list longer than the ones for RM).
OK, let's say in that every single kingdom of Dominion that has any of the 14 cards you've both mentioned, you would prefer Ruined Market over Copper 100% of the time in all circumstances. This is ridiculous, BTW.
That's less than 47% of all kingdoms.
And do we agree that Copper is better than RM in all other cases pretty easily? I feel like we've given Ruined Market every single advantage we possibly could here and this is as good as it can possibly get? I can even make pretty strong arguments for why Copper is better than RM in all decks for Prince, Graverobber, Procession, Transmute, and Venture (though "all decks" is surely too ambitious. OK, "almost every deck").
But this is exactly the point I'm trying to make. This choice is so clear to me because I'm thinking about this in a way that could possibly be practical, and it seems a lot of people really like to talk about edge cases and it's actually affecting peoples' judgment. Yeah, it's really cool when we can take a card that is *much worse than Copper* and figure out how to make it a hero, and it's lots of fun to talk about, but remember this?
Generate 20 random boards and I challenge you to make me an argument for any deck you could build on any of those boards (and have a decent shot at winning) where Ruined Market would be better to add than a Copper.
Looking at Rated-Game-style boards (which the community accepts as being the best way to measure overall Dominion skill if I'm not mistaken) and finding how many of them this would actually occur on seems to be the best way to answer this question (unless someone can suggest something better) and that just seems to be glossed over.
The only argument that holds any weight for RM in my opinion is the one where "there's a good engine and no +Buy." Unfortunately, the premise of this question doesn't apply to actual Dominion games, it seems, because the best-case-for-RM assumption that you can just gain it whenever it suits you seems to be what we're working with. For actual Dominion advice,
here's a helpful thread, BTW. It would be useful to actually generate kingdoms and see how often this happens, but even in that case, even if we assumed RM was game-changing and amazing in those cases, I don't think it happens often enough to really consider, let alone outweigh the fact that it's *so much worse* than Copper pretty much all of the rest of the time.
What I'm trying to say is that things that are fun to talk about and things that actually win you more games of Dominion seem to be getting very much clouded and this poll and discussion reflects that strongly.