The Chinese second-hand economy runs largely through
闲鱼 (Xiányú, Alibaba's Idle Fish),
转转 (Zhuǎnzhuǎn) and WeChat groups.
二手交易平台 'second-hand marketplace platform' is a fixed phrase. The activity itself is sometimes shortened to
二手 in compounds:
二手书 'used books,'
二手车 'used car.'