Posting in another thread, I realized that there's something I now look for in a vintage machine- the paint job!
You can clean, fix and refurbish just about anything on an old machine, but if the paint is chipped or flaking, you've probably got trouble.
For a rare or collectible machine, maybe you won't mind. Or maybe you find a great old 15 in a beautiful cabinet - stripping and repainting it would be fun for you.
It's hard to see in auctions and the first time I bought a machine on eBay, the seller nailed me on paint condition. She hid the damage to the paint by careful photography and strategic placement of the items she was sewing and the items that came with the machine. The areas to look at carefully are the edges of the bed and the handwheel.
Most sellers are up front about this damage because it's so common.