New information technologies allow for the design of innovative electronic markets which can improve the efficiency of trading. In this context, several internet marketplaces have recently extended the flexibility of their selling mechanisms by creating hybrid institutions which combine an auction with a fixed price offer so that buyers can choose to bid in the auction or to acquire the item for the fixed price.Applying a Market Engineering approach, this book introduces a model of an auction with a posted price offer and investigates the characteristics of such mechanisms. It discusses the respective equilibrium strategies of both the sellers and the bidders which provide useful insights into understanding actual behavior. The theoretical results are compared with strategies of students in a controlled experiment. The experimental observations indicate shortcomings of standard economic theories and help to further improve electronic markets.