Linguistic diversity in heterogeneous communities, such as in markets, can trigger code-mixing events that are important to study. The purpose of this research is to describe the forms and factors contributing to code-mixing in buying and selling activities in the market and to apply the findings to the teaching of the Indonesian language in high school. The research approach used is qualitative with a descriptive method. The data in this study consists of utterances containing code-mixing. The data sources were interactions between sellers and buyers at Simpang Sribhawono Market. Data collection techniques included observation, documentation, recording, and transcription. The data obtained were analyzed using an interactive data analysis model. The results of the study indicate the occurrence of code-mixing in buying and selling transactions at Pasar Simpang Sribhawono. The most commonly found type of code-mixing is in the form of words, followed by phrases and clauses. This code-mixing primarily occurs due to situational factors. In addition, a secondary factor contributing to code-mixing is the use of terms considered more appropriate and commonly used. The findings of this study are implicated in the teaching of Indonesian language in Phase E, as high school students generally have knowledge of the diversity of regional languages in Indonesia as well as foreign languages. This implication is applied in the teaching of negotiation texts, emphasizing linguistic aspects while also serving as a concrete example for students to practice writing negotiation texts.