Offer / counter-offer? Audiogon rules

One thing I would avoid is dealing with people who want to contact your via "in mail" on the site, then want to negotiate outside of the formal Audiogon "offer" method.

I had a guy who said he had all sorts of questions about the unit I was selling, asked for my phone number, called me, we settled on a price, then when he arrived at my home to pick up the unit, said he thought about it and decided to offer me $600 less.  So I sent him packing.

as others have said, once offer declined, no obligation either direction

I would simply write and ask if they want the piece, and if they still want it for their offered price. 

You could say you realized you were being needlessly rigid.

Get ready. If you contact him again saying you’ll accept the original offer, 100$ says he offers less the second time. He’ll think you’re desperate. Good luck 

Fishing is a tough business.  It's a big ocean and fish can be hard to find.  Once you get one on the line, get him into the boat then you're done.  Let him get away and you don't know when the next one is going to come along, and it's just as likely as not that it will be smaller.