A couple things spring to mind.
First: if it is impossible or at least VERY hard to have direct social and economic pressures to conflict and grow it would push groups to compete or gain advantage via bonding via trade or raid with external groups. this would seem to help your trader model
Secondly: so we ask what you need to support it. the first thing I would come up with is that birth groups are bonded to certain lands/uses. Just like Japanese villagers were linked to certain lands and the village was expected to produce certain taxes or an Indian caste was expected to live off its given assigned work indefinitely. now if all the INTERNAL support (arable land, forest, etc) is taken but seafood and external trade is traditionally NOT assigned then any group that wants to better itself, once they provide their taxes will have to do so via external trade. Small groups may find it useful to have loose associations to support things like embassies et al.
Third: So in order to have the above you need an authority potent enough internally to make the birth roles stick. Could be religious or an imperial court.
As for clothes etc, what kind of climate are we talking about?
edit: (and wow my spell checker works in this browser)
I would suggest looking into the early SE Asian kingdoms as they were rather mercantile. Srivijaja,, Funan, Lavo
for linguistics I'd actually recommend Korean. Words tend to be shorter and the syllables fit together is a comparatively smooth way for untrained anglophones. This means they are more likely to be actually remembered and used by the players.
also what degree of magic are you planing in the world in general and in the culture specifically. Because this can radically change things. If for example magic is quite common the economic power of salt (which otherwise these islanders could be in a prime position to supply) could be massively lowered as the more inland groups wanting it for preservation can turn to magic instead. Or for example if there is strong social pressure for wizards to create permanent walls of fire as some sort of customary display of mastery, taxes, etc. those walls could well be used for kilns, glass making, salt production, smoking fish, and other exportable goods. Sure those high level wizards may be rare but that is infinite fueled fire that should last as long as the permanency spell allows so they could build up in regions that are stable and each could be shared by many craftsmen. And that is without even getting into weather information/control spells.