I'm offering no legal advice so take with a grain of salt...but if your guns were to be stripped lowers only...and you happen to sell the remaining parts to the gun in a separate transaction 5 mins later... it would not be an "assault weapon" per WA i-1639 BS. IIRC stripped lowers are not a pistol or rifle semi auto, or otherwise until they are assembled. They are just classified as a "firearm" until the rest of the parts are attached to it. (note this only applies to AR's that were originally purchased as a stripped lower.)
So if your buyer is not against assembling the weapon, I would imagine that may be a way around it...but I would consult one of the FFL members that are more well versed in the local firearms laws regarding that.
Otherwise a complete semi-auto rifle would be considered an "assault weapon" to WA and require FFL transfer, BG check, registration, sheriff/CLEO signoff, and mandatory waiting period just like a handgun does in WA state.
So if your buyer is not against assembling the weapon, I would imagine that may be a way around it...but I would consult one of the FFL members that are more well versed in the local firearms laws regarding that.
Otherwise a complete semi-auto rifle would be considered an "assault weapon" to WA and require FFL transfer, BG check, registration, sheriff/CLEO signoff, and mandatory waiting period just like a handgun does in WA state.