【长难句】
But in some cases, one marketer's owned media become another marketer's paid media – for instance, when an e-commerce retailer sells ad space on its Web site.
【译文】
在某些情况下，某营销者的自有媒介会成为另一个营销者的付费媒介。
【析句】
主句结构清晰易辨，one marketer's owned media become...paid media,破折号后，when an e-commerce retailer sells...则是when引导时间状语后作插入语。