Annoying as the practice of backing out or disappearing may be, I disagree that there is anything legally binding or contractual contained in a potential buyer's making an offer or claiming they will buy an item. Auction bids are often described to be considered this way, although I don't really believe that this principle would hold up put to the test, but with regular classifieds, an item isn't truly sold until the money is in your hands.
Permit me to offer a little bit of professional perspective, since I have spent most of my life's work so far doing commissioned retail sales. The absolute, most surest sign that you are never, ever going to see someone come back through that door again, is if they confidently proclaim that they definitely are going to buy an item and will be back to get it soon. If all the salespeople in the world sued every time this "contract" was broken, there wouldn't be enough lawyers in the world to litigate it all.
And just so no one misunderstands, I am not blaming or chastising customers for behaving this way at all. Of course a small minority of customers might actually return and really buy, but in general, the words "I'll be back" are widely understood by professional salespeople to be pretty much directly translatable as "No" (but we know that's when our real work is just beginning). Despite that most of the time, the no-sale is not the customer's responsibility but rather the salesperson's, the "I'm going to buy it tomorrow" approach simply represents a more comfortable way of communicating a negative sentiment for many folks than a direct rejection. Customers changing their minds or saying what they think the salesperson wants to hear will always happen, and there are always underlying reasons for it, although even a good salesperson may in some instances never discover exactly what the reason is. Just human nature and a fact of life.
Anyway, my point, as it applies to Audiogon, is that since a sale usually can't be consumated in real time, and communication isn't truly continuous or complete during the selling process, the seller's ability to influence or ensure the outcome of a transaction is necessarily limited. There is simply no way of accomplishing the equivalent of a customer's saying "Yes, I'll take it", making the payment on the spot, and taking the item away with them. The time and contact gap can be fatal to the deal. Selling anything is like trying to hit a pitched baseball - you must be prepared to fail a majority of the time in order to be successful in the end. Frustrating, but true. Again, Bob's commentary about not losing any sleep if you can is still the most realistic advice you're going to get.