The rules are written as if you must do all trades first, which would include ports, then do all building.

However, in all official tournament play, including the World Championships, you are allowed to build and trade in any order.

So it is difficult to answer the question properly, but I would advise to allow building and trading interchangably. I think that the rules are written in the way they are for ease of teaching the game.

I agree with Mr. Clark that trading and building should be interchangeable. Given the lack of any appreciable time scale in the game (i.e. is each turn a month, a year, a decade? Who the heck knows?), and for that matter, the lack of a distance scale (just how big are those hexes, anyway?), it makes little or no sense to rigidly prioritize the sequence of a player's actions. My group always plays that building and trading can be done in any order desired, and thus any new ports created may instantly be utilized.

Well, the Mayfair 3rd edition rules are written that for people's first games, have it such that ALL trades (port, player, and those with the bank) must be done at the beginning of one's turn. The rulebook then also mentions that for later, advanced games to just go ahead and trade whenever you want on your turn.

As mentioned, if you're doing the former, then no. You gotta wait till your next turn to use the newly accessible port. If the latter, you may use the port right away, before you end your turn.

A player did the former once and it annoyed me. I planned on building a road, then I couldn't build a sett b/c I forgot to trade me 3 sheep for one brick. Since I built a road first, it was already too late. I left the trading phase. I was used to playing online on Sea3d where even if someone sets the house to the former, it just means NO PLAYER TRADES. You can still do bank and port trades as needed to build after you're done with all your trading.

In the original official game, you must first Trade and then Build. Since it's actually rather clumbersome to lay your resources aside that you're going to use for building, it's easier to just mix them up. However, if you want to keep the rules in mind, that means you cannot use harbors that you gained during the same turn.

Of course, you can houserule it away. Also, C&K/S&R changes the order of your turn a little (no cards before rolling the dice, save for alchemist, trading and building together are one phase), so it's not an issue there.