The best way to monetize extra pixie dust is probably to make Greater Healing potions. A stack of 99 pixie dust, 33 crystals, and 100 sand/50 glass makes 363 (58%) profit over the sale price (mostly from the crystals) of the ingredients. Use of an Alchemy Table will increase the yield by 50% (about 5 of extra profit).
Wings are also profitable, but the various "wing tokens" (which can only be used for wings) provide competition for the Souls of Flight.