I'd tend to avoid buying used long underwear or wool socks.
The "cheap, serviceable, but used" is... fairly picked over by the locals (and especially the homeless).
Consider San Francisco for a moment and the daily occasion of tourists who suddenly find that the fog rolling in the evening is cold and then buy the "cheap" (but horribly marked up) jackets that aren't the right size in which they stand out like sore thumbs.
Or they could go to the Salvation Army store and buy a winter jacket - consider the availability of them there.
Or they could go to REI or Farm and Fleet (I know there aren't any F&F in SF) and buy a new winter jacket there.
After childhood, locals have winter clothes that often last 5-10 years or more (and they pay for durability). My winter jackets are from '10 (I've got a medium weight photographer's jacket that I got in Keeble & Shuchat in '01 that I still wear... wish I could find something like it again). My father's winter jackets are in the 10-20 year range.
I'm also going to note the plural. Locals will often have two or three "winter" jackets depending on the weather.
The only time you get a jacket for a season that is cheap is for a child who is going to outgrow it by the next winter.