Currently, a player is allowed to trade after building,
both with other players and via "maritime" trades with
the bank (whether via a port or via 4:1 exchange).
However, trading should be disallowed once building
begins.
According to the SOC Settlers' Almanac (under Trade,
pg. 17), "A player must finish all trades prior to building.
A player may not trade during or after building."
This also applies to trades done through a newly settled
port (3:1 or the resource-specific 2:1 trading).
Again, from the Almanac (FAQ, pg. 18):
"3. I just built a settlement on an ore harbor. Can I
immediately use the harbor to trade two ore for another
resource?
No. Building is done in the building phase; players must
trade before the building phase."
-K
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The game is based on the FIRST EDITION Mayfair rules, which
are different from the German rules and the third edition
Mayfair rules which you quoted.
The first edition Mayfair rules do not prohibit trading
after building. They are like Cities and Knights in that way.
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Actually, according to catanonline.com (yeah, its an MSN
site, but hey) they state that when conferring with German
Principals, the trade/build cycle is more flexible and you
can do both interchangably.
Here is the quote (http://catanonline.com/news/Devdiary1.htm):
"...we also had to modify how newly built settlements/ports
were handled. Using the earlier turn sequence, all trading
had to be completed before any settlements/ports were built.
So, a port could not be used on the turn it was builta
factor reflected in our design and prototypes. However,
using the newer, flexible turn sequence, a player could
build a settlement/port and then make a port trade using the
newly built port. The question then became: can you trade at
a port the turn it is built? The answer from the German
principals was that you could."