Her main challenge was selling the concept of remote work to a traditionally office-bound team.

selling 是动词 sell 的现在分词形式。sell 的基本意思是"出售、销售"，在这里引申为"推销、说服接受"的概念。selling the concept 表示"推销某种理念或想法"，即让他人接受某种新观念。