How hard is it to import a car to Seattle from Vancouver, Canada?

Not difficult, but mildly complex:
Start with vehicle age.

More that 15 yrs old: no special import paperwork required. Buy car. Get title. Wrinkle: Seller needs to fax title to US border at least 72 hrs before crossing (checking for theft, etc.) Have insurance coverage from your Canadian provider. Drive to border. Show receipt, pay tax. Enjoy*

Less than 15 yrs. old: You need import paperwork from RIV-Registrar of Imported Vehicles. It will add a few hundred in costs to the process. You will still get seller to fax title to US Border at least 72 hours prior to crossing. Same insurance needs. Then, once in Canada, you must get vehicle inspected and brought into conformance with Canadian safety regulations. This will include any needed repairs to existing systems and likely require adding daytime running light module. They also add a sticker to the imported vehicle door jamb with needed info for licensing and re-sale.

Well, as far as I remember it is not a big deal. I was using https://clearit.ca/services/importing-cars to order a car from Toronto. They provide a really good service that helps to import a car, do deal with all different documents and custom. I didn't have any problems with all documents and save my nerves.