Antonyms for the phrase "buy in on" could include "dissent," "disagree," "oppose," or "reject." These words convey a sense of disagreement or opposition to an idea or plan, rather than the agreement and acceptance implied by "buying in." Choosing to dissent, disagree, oppose, or reject a proposal may be useful in situations where the proposed plan is flawed or goes against one's values or beliefs. Alternatively, using antonyms to "buy in on" may indicate a lack of interest or investment in a particular idea or plan, rather than outright opposition.