If the guitar is in reasonable condition, then my guess is that $650 isn't an unfair price - but you should haggle! Martin seems to have been more prominent in the tenor guitar line than Gibson - particularly in the 1920s - and my guess is a Martin of similar vintage to the Gibson would be a better bet. I had a 1964 Epiphone (Gibson-made) Texan 6-string, which was great when I bought it, but eventually the face (cedar) 'died' and it lost depth and projection.