There may or may not be a clear definitive ruling on this in the rulebook, but it's something that keeps confusing me in the actual play, so I'll ask anyway. Here are several questions that, when put together, should illustrate the points I'm having difficulty.
I play a card that gives me one coin for each brown card in front of me or either of my two neighbours. I personally keep forgetting whether this counts cards that came into play this turn, but a quick check of the rules summary clarifies that yes, these cards are counted.
I mistakenly play a card that I (and my neighbours) have insufficient resources to build, when I look closely. Obviously I cannot play the card - but I take it I can still discard it for 3 coins, or to build a section of my Wonder?
I mistakenly play a card that I (and my neighbours) are short of resources to build: let's say short by one stone. However, it fortuitously happens that one of my neighbours has built a one- or two-stone quarry this turn. Can I pay him the appropriate fee and build the card? Or can I still only discard it for coins or to build a section of Wonder? My first suspicion was that all building happens simultaneously, so I can't use materials that aren't available at this point - but if the card mentioned in my first example derives benefits from cards played this turn, why can't this?
We could get all MtG on this and break it down into discrete phases: the buy-resources-from-neighbours-phase, the build-with-resources phase, the reap-benefits-from-just-built-cards phase. But that all seems a bit anal for Seven Wonders. Is there an easy solution to the problem of looking left and right to see if your neighbours have the resources you need, and having to ask "errrrrr... so did you play that quarry this turn, or not"? (Apart from not being so scatterbrained, obviously!)