Am I justified in my frustration?

Rusted stuck or not, full of mouse **** or not, frustrating or not, what he sold you was a rebuildable core. At least judging by your pictures that’s what you got. I see no reason why this could not be rebuilt and used. To quantify the amount of time and skill involved in rebuilding is a mute point. So is the price, as that is between buyer and seller. But a $250 rebuildable 8-3/4 is what he sold you and exactly what you have. Hell at $350 that’s still a “market price” deal.