I'm not sure whether this goes into the bazaar or GD, but since I'm not actually selling anything, I guess it fits better into GD.
Anyway, my friend likes to play Spiral Knights, but he doesn't have time to play it that much. Being somewhat obsessively compulsive, we both agree its a shame to let his mist overflow, so I sometimes play for him, but I'm not all that free either.
I heard some people use mist to "reduce" the costs of upgrade slots, and then sell them at slightly cheaper prices, netting them some CE. Thing is, I almost never see anyone doing this, so I'm wondering, how likely are people to buy them? I don't want to buy some extra upgrade and then waste his CE.
You're better off giving him a 2-star recipe and some mats, and crafting two items a day.
Most popular 2*s sell for ~2k on AH, meaning you get a ~4k return on your daily 100 mist + 800cr + mats investment. (Materials are often worth ~1k per 2 crafted items, but you probaly have tons of them collecting dust, and they're a hassle to sell.)
That works out to approximately 2k profit for doing little more than logging on. And once in a while you get a UV that you can sell for more. It's probably a lot more profitable than reselling slot upgrades, especially with CE prices nowadays.
In my experience, the wolver, jelly, and magic sets, and owlite are all reliable sellers.