I got a pretty mad call this morning from the losing side. I just explained that I set the BIN and whoever got to it first would be the winner.
I deal with endusers very frequently, and I've found that the optimal way to proceed when you have two interested buyers (assuming companies are of roughly the same size) is to quote the same BIN to both simultaneously (usually a slightly higher BIN than the one you would have cited if you had just one interested party) but do not provide any indication you're negotiating with another buyer as this might provoke them to back off. More often than not, either one or both candidates will counter-offer with a differing lower amounts, you negotiate each one up to their max, and agree to the higher bidder's price while simply telling the other no thanks. Now, is there a risk of both candidates agreeing to the BIN? Absolutely! So as soon as the first prospect accepts, you IMMEDIATELY fire the second one an e-mail which looks something like this:
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Hi ...,
I wanted to let you know that we have temporarily decided to take XYZ.com off the market. On reviewing our budget, we have realized we have completed enough domain sales recently to cover our upcoming quarterly expenses and are hence suspending our liquidation for now. We will be sure to let you know once XYZ.com becomes available for sale again as the name clearly relates to your business.
Take care,
...
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You want to avoid retracting offers AFTER a buyer accepts at all costs as this represents extremely dishonest business practice and might prompt your buyer to take revenge against you (think: RipOffReport). An an e-mail like the one above helps you avoid that potential denigration.
If you're dealing with two candidates of very different sizes (e.g. a $150K/year business versus a $1.1B/year corporation), you would take a different approach.
Hope this tip helps you down the road!