For the most part, it's hard to give away TV tubes at a typical swap meet. Millions are still in circulation and many have no use outside the dinky circle of TV collectors.
There are rare exceptions, but you're not likely to find them in the case where, "Grandpa retired from the radio/tv business and left behind a basement full of tubes."
If you do stumble across Grandpa's basement, I'd be tempted to rescue them because they aren't being made any more. But only if I had ample storage space and didn't expect to make any money selling them for another few decades
Phil Nelson