A key tactic more often employed by Flash Sale sites than Group Buying sites is scarcity, but there is more to this tactic that meets the eye.
Stock limitations can be a condition, i.e. we really only have 200 available in our warehouse, however setting a stock limit can be a very effective tactic to drive up sales, this is the Scarcity Game Mechanic. To set an appropriate limit, forecast the likely sell through of a particular item, then set a stock limit that is 10 – 15% higher than that forecast, the Scarcity Game Mechanic if played right will ensure the limit is reached and your forecast is exceeded. 
To illicit the right response, the shopper has to feel some sense of urgency due to the stress that comes from missing out, clearly this is less effective in the early phase of the sale but that’s ok given the forecast amount would have been reached on its own. Having a genuine countdown display is key, either literal or abstract, regardless though it has to be genuine, once the limit is reached the item is “Sold Out”, it’s tempting to find and release more stock however once this occurs, the scarcity mechanism will be discredited for good. 
Promoting the Stock Limit though display advertising or EDM is also good, however the level of promotion provided has to be proportionate to the Stock available. Lastly, if a Consumer comes to the site after the limit has been reached, this is a great time to reinforce the “don’t miss out next time, be first to know with our SMS program” or similar engagement/re-targeting program.
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