Here's something I just realized yesterday. Getting the lvl 45/50 memorization badges can give your heroes and villains a "side income". It takes a good bit of inf to achieve them, but once you do, it's time to start trading!
Example: Level 50 Damage IO
#1:
464,400 - Price to buy the recipe outright from an invention table.
+ 464,400 - Price to invent the enhancement, plus salvage (but it's very common)
928,800 - Total if you had to buy the recipe for it's full price. Ouch!
#2:
232,200 - Total price to invent the memorized enhancement, plus salvage.
If you post the enhancement on the market for 350,000 - 400,000 someone will definitely buy it to save all the inf that can be involved with the first example. I've been making a net profit of 100K-200K per enhancement. It's a bit of a process, but well worth it if you want to bank some inf.