I know outdoorstrader.com was pretty popular in WA state but there were constant issues with the artificial border thing between WA and OR, having about the same laws, but the feds mandated no cross state line gun trading (which lots of people did anyway). When private party trading was banned in WA altogether, everyone figured they would just slip in to Oregon and do it with no questions asked, but instead, it got all shut down and went underground.

That, and there are a small number of "constant buyers" in Portland right now. They advertise around that they will buy anything decent, then the guns are never seen again. Either they are rich guys really hoarding stuff, the government buying the used guns off the street in a defacto buyback program, or the stuff is heading down to Mexico. Whatever it is, these guys buy it, and never put it up for sale, at least not locally.

The 80% build it yourself movement never hit much popularity in WA state. In theory the market is large enough to drive it up a few notches, but so far not a strong enough market to support any specialized shops like Ares Armor in California.