Some players do that to use the market as free temporary storage space, especially during events like Tower of Titans or Lost City of Gold where the event quests bring back a lot of items in a short time.

yeah, during Lost City of Gold, I can have up to 5 full teams going out to do a quest for the event. The quests take about 45 minutes for me and sometimes they can bring back like 30 items depending on if they brought back a full stack or they brought back stuff like runestones/moonstones, or higher quality items(which only brings back the 1), and then I use a Phoenix Feather to get their asses out of bed and go on another quest. I could have like 400-500 spare inventory going into the event, and run out of room in my inventory one day in if I didn't put things on the market and I don't always have time to sit down and sell to NPCs.

Alot of the time when at work, and quickly ending quests, starting new ones, and starting/rushing what crafts I can. In those instances I can't sell it off to NPCs that easily.