When you say "trekking bike", are you referring to the type common in Europe for touring? If so, you're going to be hard pressed to find that style of bike in the States, even in Portland. But if when you say "trekking bike" you mean any bike capable of touring, then you're going to have more options, as you should be able to find some touring bikes, mountain bikes, cyclocross bikes, or "sport touring" road bikes that fit the bill.
Generally, the best place to look for used bikes and get the best prices is Craigslist:http://portland.craigslist.org/bik/
But there will be some back and forth, and you might need several days to do it. Sometimes you get lucky and can grab a bike the same day, but since it's just ordinary folks selling stuff, you'll need to figure out date/time/place/etc.
As for shops that sell used bikes, here are a few to check:
There are other shops, for sure.
Be aware that because bikes are in such demand in Portland, the prices for used bikes are generally higher than they are in other places.