Fair to change ad from sale to auction?

Probably every Audiogon member with multiple sales, and multiple purchases can understand your frustration about selling lower than someone else was willing to pay. But, almost certainly, you can look forward to being on the buyer side, and wanting to be treated to any bargain that you earn by promptly agreeing to pay someone else their bargain asking price. Only an auction will elicit the highest possible price if you wish to have buyers fighting for your item. Abruptly changing to an auction after you get your buyer seems contrary to Audiogon spirit. There is definitely a comraderie among Audiogon members based upon trading around equipment at a bargain price. In that regard, Audiogon is serving as a hobbyist outlet rather than a strictly commercial enterprise. I think you can expect your overall experience to be more fun if you do not set the goal of extracting the absolute maximum from others, and giving the least from yourself. Your question shows you have your heart in the right place, and seeing how the membership responds should reflect a range of opinions. Hopefully enough responses will be posted so you can feel comfortable deciding how you want to proceed on Audiogon.