Ever wonder where the MG gets those filled soul gems to sell? I mean, anyone can fill the lower ones, but it takes some stones to fully fill grand soul gems.
Buffy is one of the few intrepid mages who carries empty grand soul gems from her guild, just for the purpose of filling them and supplying them back to the Chironasium for use during enchanting. She well remembers enchanting her first bow with the powerful magic from a dead bandit that she secretly captured within a black soul gem and wishes to spare other young mages with big enchanting ideas the temptation to break guild rules like she did.
During her travels, she also collects scrolls for a similar purpose - supplying the merchants within her guild. Since every necrodude is issued two scrolls, finding them for her comes pretty naturally. Oh, and her forays into the lairs of conjurors and vampires generally garner enough of the rather hazardous-to-collect alchemical ingredients found therein. She provides the excess to the guild alchemists to, again, help keep them supplied.
She generally comes out of a dungeon carrying only a pound or less of extra 'loot'. That, combined with her mare's enchanted saddlebags that can carry as much as, well. . . a horse, mean that her need to visit cities is not limited by loot; rather, the need for resupply of hair conditioner and such.