If you're going to buy vintage or older machines you need to learn how they work. What makes them tick. How to diagnose their ailments. What is their foibles. Many of them are susceptible to thread jams, those are easily cleared if you study how the machine works. Not every problem is oil related. It is fairly easy, these old mechanical machines are not that complicated. People who choose to remain uneducated about these things are easy prey to the shysters and crooks of the world.
As for ebay sellers you almost always need to figure the seller is going to say whatever he / can to hype the machine.
You can't take them at their word. If they are honest, great. If not, you'll be forewarned.
I'd bet both the 301s in the OPs post are thread jammed. Easy fix. Just study the instructions all over this forum about it.
Joe