This is a basic primer on buying used tubes from sellers that know nothing about the tubes they are offering. Also offers simple tests that any person can conduct to determine the quality of a vacuum tube.
When you look for a vacuum tube, whether it be a receiving tube or transmitting transmitter or amplifier tube, there are some basic questions you would ask the seller to determine the quality. This is assumeing the seller knows about the item there selling which is sometimes the case.
First question to ask is, does the filament have continuity. This may be checked by any ohm meter or continuity checker.
Second, is there any connection or continuity between the filament and the other elements of the tube such as between the grid or plate cap and the filament. If so the tube is no longer usable as a tube.
Third, are there any loose particules inside the tube that may indicate broken elements. If there is, the tube is no longer usable as a tube but if the tube is large enough, you might consider making it into a table lamp.
Fourth, what color is the glass. Is it very brown or clear. If it is dark and dirty looking this usually indicates the tube is well used.
Five, does  the inside of the glass look clear but there is a white milkyness inside. This indicates the tube has lost its vacuum and is useless as a tube.
And last, is the tube new or used. If you see multiple evan spaced marks on the outside , even if the appears new,  this indicates it has been in a tube socket and is probably used and not new.
Thank you and good hunting from "unclegeorges"