She’s totally buying it 这是一句美国俚语，在这里buy=believe，是相信的意思。
例句：
He said he could help me get a job at his company but I don’t buy it. He doesn’t have that much influence.
他说他能在他的公司帮我找到一份工作，我才不相信呢。他在公司没什么影响力的。
She told me she loved me but I don’t buy it. I saw her at dinner with her ex-boyfriend last night.
她说她爱我，但我不信，那天晚上我还看见她和她的前男友在一起吃饭。