This study analyzed the relationship between seller trust and price premiums in open markets using actual transaction data from Auction, a leading Korean open market, based on the question of whether high seller trust can always lead to price premiums, and verified the moderating role of product price attributes. To this end, a trust function capable of measuring seller trust in any open market was derived through Auction's seller trust indicators and a web survey, and seller trust levels were calculated by substituting Auction's seller-specific trust indicators into the trust function. Four products traded on Auction were selected, and price premiums were calculated as the difference from the average prices of five domestic shopping malls. The empirical analysis results showed that the relationship between seller trust level and price premium differed according to the price level of the transacted product, and the hierarchical regression analysis results also demonstrated the moderating role of price, showing that price had not only a main effect but also an interaction effect on the relationship between seller trust level and price premium. Therefore, when trust in an open market site has been established at a certain level, price becomes a more sensitive factor in purchase decision-making than seller trust for standardized low-priced product transactions, whereas seller trust has a greater influence on high-priced product transactions. This implies that the greater the perceived risk in a transaction, the higher the price premium rewarded to trustworthy sellers. This study is significant in that it presents a more realistic and accurate role of trust in price premium formation by investigating actual transaction data from open market sites.