Deck Building questions - purchase/build & stairs

I played a 4-player game of Deck Building tonight with some new players and some questions came up.

Suppose you have 4 screws in your hand. You buy a card that costs 4 screws. Clearly you cannot buy another card. But can you still build a card from your hand? The rule book does not say otherwise, so we've been playing that you can.

A question came up about what stairs were worth for purchasing other cards. Re-checking the rule book now, I see that it does mention that stairs are not worth any screws.

The Grills if you have them in hand aren't worth any screws either.  

Why wouldn't you be able to?

Because in most deck building games that I've played, those cards would have been considered to be already "used up" for the turn. A strict reading of the rule book indicates that you can still use them, but a confirmation would be nice.

Another thing I noticed was the rule book specifies the requirements for purchasing twice in a turn, but not for purchasing 3 or 4 times. It would make sense that the total screw cost of all cards purchased may not exceed the total amount of screws in your hand. But by a strict reading of the rules, that's not the case.

Fair point, and now that I think about it, I had the same question at first. FWIW, I had it confirmed by Christopher, Adam, and Paul when they taught me.

Using the screws to buy cards does not mean the card those screws came from cannot be built.

Thanks for the confirmation.  

Thanks Christopher!