Should I focus on buying mods or farming them from battles?

Another good note, make use of the lock feature. It's fantastic and the mod won't even show up in the sell screen. Even if you accidentally hit "sell all" you won't sell your best ones.

@TVF I've leveled grey mods to 9 when I'm in a pinch or modding low-use/niche toons. Found a few +4/+5. Depends what your unique credit situation is, of course.