If you have Item mall points buy items the higher level players need and sell them in booth for game silver.
buy a mining or fishing pass and sell the items you collect
(mainly black pearls and lvl 10, 11, 12 eagle wood and cold iron from fishing)
(fine iron ore and a few others needed for compounding the higher items ... go look at material exchanger NPC in Seville)
but highest return sems to be certain items like serpent stones dirrectly bought then sold.
Make and alt on a new account with a different email so that the alt can sit in sales booth while you play your main character. Same if you make a mining or fishing character (you will need a new characters warehouse space for mining materials/ ore/ metals).
This version of the game is designed for the person that can play at least 8 hours a day (non item mall)... and it definitely designed for those who can spend a lot of real money (if you want to be a top player).
Get in a good guild and do your daily Athens/sanctum, daily London, daily trade quests every day.
Guild mates will help you complete the higher quests you cant do yourself, often share trade trends (business spy card usage)... and can help you decide what you should upgrade and in what order.
Also focusing on Generals can help you improve strength without spending silver and actually give you stuff to do which are more fun than HI all day
Beside that always try to check out what ppl need. Often high lvl players dont want to bother with farming quest cards or explo items and you can do that and sell to them and if they really want something they pay absurd prices .
In this game either need have good sense for the market and it demands or need be playing very much (including lot of HI) or need spend lot of IM.
Thats 3 options to get strong i think thats enough
However i agree please add some other source of silver income e.g. 200% more silver drop from Bimini NPC or better drop rate of CO in HI.....