To be honest, as a buyer I would hesitate on buying a used barrel in the current market. As noted, they can be had pretty inexpensively brand new, right now. This is especially true if you're just looking for a blaster grade barrel.
Then it gets down to why the person is selling. Did they just want a different barrel and are trying to recoup some of the cost? Or are they dumping a crappy barrel? As a buyer, I have no good way of knowing if the used barrel has really seen "only 200 rounds" or if it's 20,000 rounds.
So unless it's a screaming deal for something I only intend to blast with (the $20/$25 barrels mentioned above fall into this category), I would probably pass.
As a seller, I would want to get around 75% of the price of new. That's not likely, for the reasons I posted above. It's worth more than $20 to me, so I'd just put it on the shelf and keep it for a rainy day. (And it would be an excuse to build another upper / rifle at some point).
Now if it was some kind of match barrel, and I could go shoot it to verify, that might be a different story. Of course, AR15 uppers are cheap. In that instance, I'd want to just buy the complete upper and not bother with pulling the barrel off the seller's upper and installing on mine.