When complex procurement projects are conducted, it is often not possible to write complete contracts. As a consequence, the relationship between buyer and supplier is important for the success of the project. In this paper we investigate the claim that auctions in procurement can be detrimental for the buyer–supplier relationship, which is in line with the observation that reverse auctions are less frequently conducted if projects are complex. A poor relationship can result in a decrease in trust on the part of the buyer during the sourcing process and an increase in the supplier’s opportunistic behavior following sourcing. We consider a setting in which the winning supplier decides on the level of quality to provide to the buyer, and we compare a standard reverse auction and a buyer-determined reverse auction, both analytically and in the laboratory. We find that the buyer-determined reverse auction can perform better than the standard reverse auction from both the buyer’s and the suppliers’ perspective. In a buyer-determined reverse auction, it may be optimal for the buyer to select the supplier who submitted a higher bid, which may in turn induce this supplier to deliver higher quality. Standard auctions, however, yield lower prices but reduce cooperation. The degree of trust, as reflected by a larger number of transactions and a higher average efficiency of trade, is significantly higher in buyer-determined reverse auctions. Theoretical reasoning based on other-regarding preferences organizes our data well. This paper was accepted by Serguei Netessine, operations management.