Personalized advertising has become a major driver of the online advertising market. However, empirical data on the domestic market's structure and business practices remain limited. Specifically, there is a lack of detailed analysis on how numerous small and medium-sized enterprises (SMEs) utilize data and compete within a market structure dominated by large platform companies. This study explores the structure and current status of South Korea's personalized advertising market from the supply side, aiming to provide foundational insights for both academic and policy discussions. A sequential exploratory mixed-methods approach was employed, combining in-depth interviews with 13 industry experts and an online survey of 68 personalized advertising firms. The total market size, estimated by synthesizing external data and expert opinions, was projected to grow rapidly, from KRW 2.37 trillion in 2023 to KRW 3.57 trillion in 2024. The survey results, primarily reflecting the SME ecosystem, showed that retargeting (30.1%) was the most common advertising method, with a majority of firms placing ads on third-party websites (53.9%) and relying heavily on third-party behavioral data (64.4%). Key identifiers for behavioral targeting included cookie IDs (61.8%) and device identifiers (63.2%). Firms cited 'targeted exposure to specific audiences' (57.4%) as a major advantage, while expressing concern over 'increased data collection needs' (42.6%). The study contributes by providing rare empirical insights into market scale and specifically highlighting the practices of SMEs, which have been understudied. It also suggests policy directions such as enhancing transparency in data use. A key limitation lies in the small sample size (12% response rate) and lack of participation from dominant platforms, reflecting the opaque and concentrated nature of the market. Future research should expand sample diversity and include user perspectives to better balance advertising innovation with data privacy.