Well, an O1/Wfd is 61 keys, with the O1/W Pro at 76 keys and the O1/W Pro X at 88 weighted keys (the other two are synth keys).

I sold my O1/Wfd for $1000 in March or April 1999 (which was a fair price back then). You can check internet sites for what's reasonable to the market, but I'd probably offer him $500 - $600 (the technology is 10 years old or so) and work my way up from there (if you need to haggle).

Peace.