Regarding vehicles, Use Tax would only come into play if bought and owned less than the previous 90 days. And ID has Sales Tax, so even if you bought a new vehicle, you would get credit for the ID tax paid and just owe the difference.
http://www.dol.wa.gov/vehicleregistration/moving.html
RTA Tax: This is the one that makes your car tabs much more expensive. It is 1.1% of the car's value, with the "value" determined by some bullshit depreciation schedule that is as far separated from reality as you can imagine. Here is the (current) RTA boundary. I imagine that over time, places like Snoqualmie and North Bend will get the shaft after promises of "high speed light rail to your town" are swallowed.
https://www.soundtransit.org/sites/default/files/documents/pdf/about/stdistrictmap07_10.pdf
NFA: Non-explosive DD's, AOWs, SBRs and Suppressors are all good to go, either via trust or individual ownership. MGs, SBSs and exploding DD's are a no-go currently. Just remember to file a 5320.20 for the SBRs before crossing state lines. You could file 5320.20's for the suppressors and AOWs, but the ATF says that is a "nice to do" not a "mandatory" action.
UBC: Yes, I-594 imposed UBC on all transfers, so to be in legal compliance, all firearm transfers need to go through a dealer. The state exempted the DEALER from having to collect the Use Tax, but I don't believe they actually exempted the buyer from the responsibility of PAYING it. If you only buy/sell guns currently from/to private individuals in ID, then you might want to round out your collection before moving. If you buy guns from dealers in ID, why would buying a gun here be any different? (Other than our higher sales tax.)
Gun Laws: The state is just one initiative (or one seat in the Senate) away from going full-retard on an "Assault Weapons Ban" and magazine capacity restriction. We fight it every time and have won most of the battles, but the continued "Californication" of Western Washington, Activist judges, and the general liberal mental illness that comes from increased population density, is the writing on the wall. In theory, we should be well insulated from such stupidity because of the 2nd Amendment and our even more cut-and-dried Article 1 Sec 24, but that relies on our courts to be honest and do their jobs correctly. Based on the Supreme Court allowing the Seattle Gun Tax to stand, it won't be long before our rights are really screwed. If not outright banned, then we'll have exorbitant "gun taxes." One candidate is already saying the Seattle Gun Tax should be doubled. Since the court has said it is within the City's rights, why not make the tax $1,000? $10,000?
Anyway, not trying to scare off someone from the area. The reality is that we need more conservative 2nd Amendment supporters to move here, in numbers and faster, than the liberals. "Cut and run" is a guaranteed method of ceding more territory to the enemy, and eventually you will be surrounded and overrun. Even Montana has some blue counties now......