Excellent insights on the risks in Groupon's sales model from Wharton professor, David Reibstein. Here's the important bit:

Knowledge@Wharton: Why do you believe that business model will not support the current growth rates? Reibstein: Let me talk about some of the fundamental weaknesses. Obviously, one is, however brilliant of an idea it is, there is also now a huge increase in competition. When Groupon had few competitors, it was more viable than it is now with 499 competitors. But that is not the big weakness. The Groupon business model works better during a recession than it does during a vibrant economy. I will explain why, and this is where it gets intriguing. The reason some retailers might be willing to provide supply to Groupon is because they have excess inventory. That is particularly the case for services. One of the services I notice frequently [offered on group buying sites] is that of beauty salons. They have so many seats and so many beauticians. If I don't sell that 3 p.m. to 4 p.m. time slot on Thursday afternoon, I cannot carry that time slot in the inventory tomorrow. It perishes. It perishes in the same sense as an [unsold] airplane seat [once] a plane takes off down the runway. Because of the recession, there has been an abundance of people who are forgoing beauty salons and other sorts of luxury, discretionary services. Rather than let that airplane seat go [unfilled] and the beautician hour go with no revenue, [companies] would [rather] sell it for a little above whatever the incremental costs are. So there is a willingness to do deep discounting. As the economy picks up and there is less excess inventory, the availability of supply will go down. The willingness of the merchant to offer deep discounts will go down. The business proposition to the customer will be less attractive if [the item or service being offered] doesn't have the same deep discount.

I wonder if Groupon is keenly aware of this and whether it has anything to do with their rush to IPO. (hat-tip to Mitch Kapor)