According to a company press release (German), the domain name Shopping.de was sold for €1,960,000 (~$2,800,000) in 2008. The domain was purchased by Unister GmbH, a company also operating other popular online portals such as News.de, Auto.de, Geld.de ("money"), Kredit.de ("credit", "loan") and Preisvergleich.de ("price comparison").
Since then, Unister GmbH has been busy developing an online shop under its newly acquired domain. After one year of development, the Shopping.de website has finally been launched and is now waiting for shoppers to arrive and spend their money. The press release says the launch of Shopping.de will be supported by an extensive marketing campaign, although I haven't seen or heard any ads for the site, yet.
The high sale price of €1.96 million makes Shopping.de the most expensive .DE domain name so far. Unister was also the buyer of the Kredit.de domain, which it bought for a reported €892,000 (~$1,275,000) and which is the second-highest .DE domain sale that I know of. It's sales like these that give you an idea of how many high-value domains really change hands without being reported to the public.