The receiving-end urban power grids with high load density are facing an increasingly serious shortage of regulation resource. Urban buildings have many promising and flexible resources such as central air conditioners and electric vehicles, and can participate in the supply-demand interaction by constructing the virtual power plant (VPP) through aggregation. With the rapid development of the demand response (DR) markets, the market-oriented pricing and trading of flexible resources have become a trend. Therefore, considering the benefit demand of buildings and VPPs, the bidding mechanism for the building VPP participating in the DR market trading is designed. First, according to the characteristics of flexible loads of buildings, they are divided into lossless transferable loads, lossy transferable loads, as well as lossy reducible loads, and the corresponding formulation methods of capacity-cost are proposed, respectively. Then, an allocation method is proposed to guarantee the reliable revenues of both the buildings and the VPP, so as to continuously motivate them to participate in the DR markets. On this basis, the bidding optimization model is developed for the VPP to participate in the DR markets to realize the maximization of the revenues of VPPs in different scenarios. Finally, the effectiveness of the proposed mechanism in market trading and revenues allocation is proved by case simulations.