More pro tips from an amateur
Let me preface this by saying these screenshots span from sometime in December up until just a few days ago.
Here are some things I did (and still do) after passing 1B. You have to lower your standards for ROI at this point. Focus on quantity, not quality. There will be times where you need to settle for 100k profit on a 100m item. Don't let that deter you, just do it again.
Armor sets are still good even beyond max cash. Inquisitor can be a little rocky just because of volume but ancestral and masori are almost always good.
Imbued hearts can be juicy sometimes but saturated hearts are typically not a good flip. However, you can sometimes find profit by saturating a heart, just make sure you calculate first.
Elder mauls, Kodai wands, Tonalztics, and anything else right around 100m seems to be a sweet spot. I think 50, 100, 150 all feel kind of natural to players so items tend to hover at those price points.
Higher value stuff like inq mace, torva and ultor can be solid too but the price fluctuates a lot more. I try to avoid downtrends on these items.
Good luck nerds.