Although the seller may not be directly culpable for the theft it may be related. Were your doors locked or was there broken glass? If not, somebody could have used a key. Perhaps not the seller, but a deadbeat kid or friend he had previously loaned it to.
I have a 85 and as far as the door goes I can use any of my toyota keys to open it....wont work in the ignition...but can get the doors open.
Having come from up there (Normandy Park) there are aot of good Cruiser people up there.....they will keep both eyes open for you
But brother I do feel your pain.......