Well, SPN Lifer brings up a whole new issue about if they can or can not tax goods brought in from the USA mainland in luggage. I'm not sure they can inspect for taxation purposes baggage from the USA.
"§73126. Examination of Baggage. The Collector may cause an examination to be made of the baggage of any person arriving in Guam from a point outside the United States of America, including the Commonwealth of the Northern Mariana Islands, in order to ascertain what articles are contained therein and whether prohibited, notwith- standing a declaration and entry therefor has been made. "
The way I read it, the customs collector has the right for an examination for baggage coming in to Guam only from outside the USA. And is that because the items coming in from the USA are non taxable??? If you go NRT / Guam you'll be under more scrutiny then going HNL / Guam or IAH / Guam I know that for a fact.
If you want to find out officially, go to www.guamcustoms.org and email them your questions. The islands are great, IMHO, but they sure can get confusing. I very well could be doing things wrong and giving you bad advice.
I'd just tell them that you are a US citizen departing the USA, transiting Guam and ask them if you can can bring gifts bought in the US mainland for friends who are residents on Guam, and if there are any fees / valuation limits / restrictions associated with gifts. Worse case scenerio, wrap them in holiday wrapping paper, I've never seen a customs official in the islands open a wrapped gift. I'd love to hear your findings or experience after the trip.
And SPN Lifer, Kosrae is beautiful, friendly, relaxing, etc... but really not much to do except the diving, eco-tourism type things etc... The most under developed island of all. Never a rush and no crowds or such, but still many many years behind even the other islands.