In my experience, marketing is the fastest way to aquire inf. Now with that said, there is a learning curve. Like Snarky said buy cheap stuff, convert to stuff that sells for more and has a demand.
You can also look at super pack flipping. Buy super pack, open, convert, catalyse, sell.
I use to make a decent amount but farming and using all the inf to buy super packs. This gave me a influx of inf with the added benefit of IO'ing out my alts.
If you go the super pack route keep in mind that you need to buy a decent amount. The reason is that you are not aways guaranteed a ATO. Some times you get 1, 2, or 0.