I'm talking your average 1960s or 70s, generally Japanese zigzag machine since they're the most common. I don't see pouring a lot of time in them because you're not likely to get much of a return.
If you study the market for which machines sell for the most-Featherweights, 301s, Rocketeers, Elnas Pfaffs, etc, and concentrate on those since the labor stays about the same for any machine then you might be able to turn enough of a profit to justify your time. The biggest hurdle is getting that more desirable machine at a price that leaves some money for you when you're done.
The antique machines are pretty much a labor of love. The general population doesn't see much value in them except as a decoration or a plant stand.
I've given a couple machines to friends but I haven't tried flipping any. I have toyed with the idea just to pay part of my hobby though.
Rodney