Originally Posted by Sprocket Man
Ask the bidder how tall he is. Whatever his answer is, tell him that he'd fit perfectly.
That's generally how it's done.
Seriously, the way most handle this is to say, "I am 6' and the bike fit me fine. This way you are being factual. You said it fit you fine. Doesn't mean it will fit him fine but if he is 6' he will think it will. Put the responsiblity on the buyer.
In your case say....."the owner was 5'8" and it fit him fine." (or whatever he is.)