Convenience. If I move frequently, I don't have the time or desire to deal with listing stuff for sale, deal with people coming to look at it, deal with getting paid for it, deal with people trying to scam me, etc. Just open my browser, a couple of clicks and I'm done with it. If it costs me a couple of hundred dollars more over the year, I don't care. It's probably just background noise compared to my income and the convenience is worth it.