I bought my first 503 off Craigslist. I bought the second one at an estate sale. Both 401s were purchased at online auctions.
The first 503 was returned to the owner when their church closed their sewing classes. If I remember correctly, I paid $35.00 for it. The photo on Craigslist just showed the back of the machine, as it was still "down" in the cabinet. The ad just said Singer sewing machine. When I saw that curve, I knew it was a Rocketeer. It was in OK shape, with a few chips in the paint. The second 503 was perfect. It hums. I bought it on the last day of the estate sale. I didn't take the cabinet, and they were just happy to get rid of it.
I didn't pay much for the first 401, but it was really nasty and gummed up. It took a lot of oiling and cleaning to get it to work. I cleaned the second one, and it works well. However, I'm still sewing on the first 401.
I'm blessed with slant-o-matics. They are my favorite machines.
bkay