I want to thank everyone for their advice. In the end, I didn't buy the treadle. The machine was very rusty. It looked to be mostly surface rust, but still. On top of that, for some reason the needle wouldn't get up and down. There was a little movement and then it just stopped, even though the wheel would keep turning. I opened the machine up to see if maybe there was some sort of blockage that was easily fixed. I could see that when the wheel turned the disk that has the needle column (for lack of the proper term) attached to it would turn about 1/3 of the way and then stop. Fiddling with the stop-motion screw didn't help. I think the problem was somewhere inside the machine where I couldn't get to it. Yes, maybe oil would have helped, but the fact that it was basically not working combined with the rust meant I didn't take the machine.
Maybe if the seller had lowered the price enough I'd have taken it, because the treadle table was in excellent condition. But this will be my first vintage machine and I really want a usable one. A little TLC - cleaning, polishing - is okay, but at the very least I want all parts there and the machine in working order. So I'll keep looking. I'm not in a hurry, so that's okay.