Even in ebay, I can offer something for a firm price, someone can offer to buy it at that firm price, and I can still cancel the sale for a variety of reasons.

Even in ebay, I can offer something for a firm price, someone can offer to buy it at that firm price, and I can still cancel the sale for a variety of reasons. If someone offers a OBO price, I can counteroffer, and still cancel the sale after they accept the counteroffer. It may hurt my eBay reputation, but I'm under no obligation to sell at any price unless I explicitly agree to it. Giving a counteroffer isn't an actual agreement to sell.