Wow, everything looks brand new! Really, it looks unused. And that is a beautiful cabinet and stool. I bought a very similar set-up in 1982 for $80 at an antique shop in Vermont. The cabinet was the same (#40 with straight legs, mahogany), but I think my seat cover was brown. I can't remember if I had the potted motor or the belt motor. It was stolen when I lent it to a friend. Up until then it was a great machine!
About the seller being willing to sell it out from under you -- who did she think you were? A passer-by? Some people put in their ad "first come first served", as a warning that they're liable to do that. There are so many flakes and people who never show. But I hope she would have at least texted you to let you know it was sold.