Can you play more than one development card?
Note that you may play only 1 Development Card per turn: if you have played this card before rolling the dice, you are not allowed to play another one afterwards.
Can you buy more than one development card per turn?
You can only play ONE development card per turn. Also, the Development Card you are playing can not be purchase during that same turn you 'play' the card.
How many times can you trade in Catan?
You may trade as many times as you can, using single or multiple cards, but you may not give away cards (i.e., "trade" 0 cards for 1 or more cards).
Can you trade development cards?
One might define Development Cards in hand as "resources" available to the player, and therefore tradable. This is not alllowed and breaks the rules. Development cards are not resources as they are not produced by land hexes. They are thus not tradable and their trade is forbidden under all version of the rules.