Well, the highest prices I have ever seen on Harmony archtops would be in the $500 neighborhood. However, these would be Cremona's and Monterey's from the 1960's.
If I recall correctly, the seller had a $1,300 BIN price with no takers. Assuming a potential buyer would be willing to stretch a few hundred dollars for a guitar they really wanted a more realistic price range for the guitar is probably $500-$1,000. I say start a negotiation at whatever price point you are comfortable with within this range and let a conversation develop from there.
The seller certainly has a rare instrument in excellent condition. However, I would remind him that collectors willing to pay big money for a Harmony might be rarer than the guitar
Good luck, let us know what happens