Situation: You come into the possession of an item worth 20ish billion zeny, although it's super cool, you don't exactly have use for it right now, so you decide to sell it for profit so you can do something else with it's worth.
You could completely gear a single class to the teeth!
You could crash the market for a specific item!
You could get a few shiny things!
You could collect a bunch of -_- you ALWAYS wanted to collect!
You could do ALL the leveling!
Me personally, I'd use the money to buy one of every type of useful garment for multiple classes (Shooter Faw, Etran, Vit GFWS, HBP) as well as collect all useful Temp boots and a Porocoro of awesome refine, and any money not spent would go into buying all the BMX3s and consumables possible :U