Okay guys. I looked it up and found RCW 82.12.010:
(6) "Use," "used," "using," or "put to use" have their ordinary meaning, and mean:
(a) With respect to tangible personal property, except for natural gas and manufactured gas, the first act within this state by which the taxpayer takes or assumes dominion or control over the article of tangible personal property (as a consumer), and include installation, storage, withdrawal from storage, distribution, or any other act preparatory to subsequent actual use...
IMHO anyone who takes possession (and dominion or control) in Sumas can claim Nooksack County as the location of first use. But I can tell you county bureaucrats have ways they have "always" done things which always to don't comply with the law as written!