How many copies of an expansion do you need to sell for it to be considered a success (or at least not a failure)?
I don't know. I can not even try to get it published and yet be happy with it myself. A publisher might have expectations that didn't get met at any level really, from "big success but hoped for bigger" to "only expected to sell 1K and didn't."
The Wikipedia article on the SdJ - citing an old article of Scott Tepper's! - says, "A Spiel des Jahres nomination can increase the typical sales of a game from 500-3000 copies to around 10,000." Obv. times change, more people are born, that could be wrong, but you know, that makes it sound like selling 10K copies should be somewhat satisfying.
In my experience those people with 3 sets would have the base set, Intrigue, and Seaside.
Which makes sense as they've been out the longest. Which sets sold the best in 2013?
Looking just at the second half of the year (thus, intentionally not seeing the initial burst of Guilds sales), the main set, then Intrigue, then Seaside.
We could run lots of tournaments, and only use new sets in them, like Magic.
Isn't that because of power creep and ensuring that fans buy the new cards?
Perhaps I don't understand you. Magic doesn't have power creep. Yes the point of me saying the above was along the lines of pushing fans to buy new cards. I have no interest in pushing fans to buy new cards; but if someone is saying, there must be new expansions, that is stuff to think about, what exactly will actually sell them. Hey, perhaps this odious stuff from Magic.
Do you think that there any enough card ideas out there that use the existing rules, or rules that are fairly easy to understand (eg "when gain")? I can't think of many dominion cards where the rulebook is doing more than clarifying the more confusing aspects of what's written on the card.
I am not sure I understand the question. Existing Dominion cards struggle to be as understandable as possible without the rulebooks. There are still new things you absolutely need the rulebook to explain - how does the potion symbol work, how do coin tokens work, how do duration cards work, what do I do with Shelters and Ruins and Platinum/Colony. What exactly does "pass" mean on Masquerade.
In fact here's another question - which printed card brings up the most rules confusions and misunderstandings when other players experience it for the first time?
Well the main set sells the best, so measured by sheer number, probably Throne Room. Possession probably gets more questions than Trader, but the Possession questions usually have easy answers ("it really does what it says") and some of the Trader questions do not.