I've seen this one listed elsewhere, although I cannot remember where. The back has been replaced with some kind of strange wood. As I recall there were some other issues as well.
That may be a 40's uke; the decal IS missing the period that the later Martin decals have. The ebony nut is also a good indicator he's not wrong about the age.
$800 for a 40s Style 0 in good shape is a little on the high side, even if it were in top shape, but a replaced back? $300 if it plays nice, but I wouldn't pay more.