Need to move a few cars

You can look at the Hagerty Valuation Tool to get an idea of what the cars might be worth. But don't look at that top number and get too excited. There is a HUGE spread between #1 concours condition to #4 fair. The price spread reflects the astronomical difficulty and cost of restoration to bring one up from a #4 to a #3 or from #3 to #2 and going #2 to #1 is totally impossible. Plus you have to recognize that Hagerty only deals with collector cars. I.e. in Hagerty's world, #1 concours means its so perfect that it should be in a museum. Even a #3 good car could be entered in a local car show and probably get a trophy while a #4 fair you'd still be proud to drive to a non-competitive event like Cars & Coffee. Hagerty's prices all assume no rust, that the car is running and driving, would pass safety inspection and is licensed for the road.

Hopefully your father's car collection hasn't been languishing outdoors all these years and could still fall somewhere on the Hagerty scale. Since they haven't run in 12 years, there's going to be some cost to clean out the fuel systems, flush the fluids, make sure the brakes work, install new tires and basically recommission them for the road. If after recommissioning they can't be any better than #4 cars (i.e. been sitting outside all these years) then I'd suggest just sell them as-is (posted here and/or FB Marketplace) and take whatever offer you can get. But if they are potentially #3 or better condition, then that's starting to get into certainly way more money than the recommissioning would cost. If that's the case, I'd go ahead and pay to have them towed to a local mechanic and recommissioned, then auction them off, either via one of the online auctions like Bring-a-Trailer or Cars-and-Bids, OR a local live classic car auction (I don't think Mecum or Barrett Jackson ever do Seattle but don't they have an auction once at year at LeMay?)

Reason you might want to spend that money up front to recommission a nice car as opposed to selling as-is, is because even though its only not running probably because its been sitting long enough that the fuel system is gummed up, you're kind of stuck with just buyers who are going to recommission it themselves and flip it. They're looking for a profit and trying to minimize risk. Your risk can't go up so you might as well put yourself in that place and take the profit for yourself. Second is that by getting it running, you can post a video online with the auction and expose it to a nationwide or even international audience. With lots of pictures and critically, a video showing that it runs and drives, people will buy it sight-unseen. If its not running, then people are a lot less likely to bid from far away since they can't be sure that there's nothing seriously wrong with it.