Nearly Headless Nick
Note : if a player has chosen a card but is unable to build either the structure
or the Wonder stage, that player is then forced to discard the card
and takes 3 coins from the bank.
Page 5 (above) of the rules states that if you are 'unable' therefore 'the cost to play a card' does not HAVE to be met (although, of course, you'd want it to - also I do not see that in the rules, maybe I missed it?)
So, it isn't as 'obvious' as we'd like it to be.
I'll just 'house-rule' - no resources may be bought on the turn they come out.
You can't buy resources that weren't there. Thematically that "year" or whatever time frame you consider each card play was taken up building that building. While a building is being built it can't be producing resources.