I think it needs to be made clear how I-594 will affect non gun owners if you want people to vote no. At the Seattle Rally, and in your statement, pretty much all that was said was how I-594 would affect gun owners. The largest population of people inclined to vote yes on I-594 are the non gun owners and they don't give a rat's butt about how it will affect us.
I-594 has no additional funding associated with it. How many tens of thousands of "transfers" of firearms between private parties occur in Washington every year? Nobody can know, but we can guess it is in the magnitude of tens of thousands with such a broad definition of "transfer" that I-594 allows. Since there is no funding for I-594 - who or what is going to cover the expense and time required for LEO agencies to conduct these tens of thousands of additional background checks? Nothing is going to fund that or provide the manpower to accomplish it. That means that LEO agencies must use existing resources, which are already stretched to breaking, to do these background checks with. So ask the person voting yes what would they prefer? Would they prefer that LEO agencies will tie up their resources doing background checks on tens of thousands of people who are volunteering to have a check done on them, who are 99% likely chance to pass it, and for which there is no penalty associated if they fail the background check because no crime is committed if they fail as long as the firearm is not transferred to them? Or would they prefer that those resources be used to investigate actual crimes and be used to catch the criminals who have already committed criminal acts?