The phrase "buying in on" can be used to describe someone agreeing with or accepting a particular viewpoint. Antonyms for "buying in on" might include phrases such as "rejecting," "refusing," "disputing," or "objecting." These terms suggest a lack of agreement, cooperation, or compromise. Someone who is not "buying in on" a particular idea may have a differing opinion or simply be uninterested in the matter at hand. For example, if a group is discussing a new project proposal, someone who is rejecting the proposal might not believe it's a good idea, while someone who is simply not buying in may not have an opinion either way.