Those look like a pretty good deal... My only concerns are that they don't say what style frames the 23x31s are for and the one in the image doesn't appear to have a serial number in the corner. This means one of two things - either they are so old that the numbers wore off or they may be left over conversion panels that weren't made with a premium mesh.
Our BSides are $10 each right now, so I'd probably go that route since I know where the mesh came from, but even "not so great mesh" at $10 a pop is a pretty great deal!