Ah, the forever asked question; Merchanting.

Buy low, Sell high.

He said it.
The more you scour the FM the more potential profit.
Scrolling wise/ Items summoning rocks are decent as are tower scrolls.
Merchanting is more of a "chump change adds up" attitude than a "SCROLL DEM SCGs"

EDIT: ALso buying smart helps.
Like if you can still find real cheap PACs and Taos from the Chest event buy them.
-Excluding ANOTHER chest event super soon- They will return to their ridculously high prices.
I have 4 PACs and 6 Taos, Just waiting til about Feb. to sell them.
*Also things like Maples Weapons etc etc do this as well*