A recent study put the question to German eBay customers who had offered a neutral or negative evaluation of a transaction:
One-third of the unhappy customers received a message that included the sentence: “I would like to apologize and ask whether you might withdraw your evaluation.” The others were offered, in place of expression of regret, either 2.5 or 5 euros “as a goodwill gesture.” Writing in the journal Economics Letters, the researchers report nearly 45 percent of those given the apology withdrew their evaluation, compared to only 21 percent of those offered cash. A direct apology: priceless.
The Dish 