Craigslist. You eliminate all the shipping costs and attendant problems and you can try the machine yourself.
I found a 301 at the Goodwill, all I wanted was the table so gave the 301 to a member here in exchange for a bit of fabric.
The 301 and 400 series (I LOVE my 404) are highly recommend because they were aluminum machines so make great carry alongs, they are under 20 pounds.
If you don't plan on doing any classes the field is wide open in vintage machines
good luck with the search. I looked for about a year on craigslist before a found my 404, there were some prior to that but condition and price were part of my watchlist.