The seller said, "Unit can power on..." It didn't "power on" for me. But it was only $31 and I don't want to pursue an eBay dispute.
I still think I can get it sorted out. I texted my cousin last night who said he'd be happy to help—he's a very hands-on electrical engineer. If it turns out I need a new power cord I'll buy one—they're easy to find online. If it needs a new foot controller, well I might like a modern one better anyway.
If it needs a new top bearing I'll have to spend the "big bucks" and get another (used) motor (I think they go for about $50). Even with that cost and the (extravagant) $35 I've spent for a tension assembly (I failed to notice it was missing a few parts in the eBay photo) it'll still add up to less than the $250 to $300 refurbished ones go for.