Is it just me or is gold too scarce for us?
I did some dozen Lightless Arbor runs without even entering the vault for I have no gold to sink into that. Sure I have some tabs and maybe more than others for I like to keep things just in case.
I imagine in MG I’d easily get several millions from just selling a few t7 items.
So what I’d like to see is a different currency for item trading so that we can generate more gold for tabs and LA.
But why would you even need gold in CoF ? Its made for not trading at all 
Because there are other gold sinks than trading.
Like the Lightless Arbor mentioned by OP.
its just you.
just take the blessing. enemies regularly drop 200+ each.
i have got over 40k gold from just 1 echo reward chest.
other than that, you can sell all uniques for between 250 and 500 each.
just take a bunch of unique amulet or ring prophecies and then sell what ever you dont want to keep.
stop imagining… imagination is not real.
if you assume you will get this, then by all means, change faction and see how easy or hard it is to get millions of gold.
you can also have another character in another faction that you can use for making your millions of gold.
You’re talking pennies 
But regarding your last point, how would I go about selling off unflagged (from CoF) items that I already have? What happens if I convert an unused character to MG (and back?)
Would it be:
after grinding echoes you have hundreds of thousands of pennies even going into multi-millions depending on various things, the biggest factor being the gold blessing.
if you are rushing through echoes and not killing pretty much everything, then you will not get much gold.
the reward chest is based entirely on the amount of enemies you kill in the echoes, more enemies more rewards.
i have 94 stash tabs and half of those were from before they reduced the price by 50%.
i have only really played CoF, because i prefer to experience the game, than just buy my way to the top.
the prompt tells you what happens when you click to leave a faction.
After changing from CoF to MG and earning hundreds of millions in a week, there’s no point to CoF unless I want to play alone and struggle… quite simple of a choice for me.
Also, the prompt tells you, you will lose all favors and prophecies, however it’s not true. You only lose prophecies from that specific character, but all favors from account (unless they actually fixed this, doubt it).
Instead of changing factions, it may be wiser to just make a new character and spend a day leveling vs losing 100k CoF favor. The dumb thing about this is… you can have a level 5 alt freshly joined a faction and actually lose everything by switching factions. Really dumb. When you can spend another 5 minutes to make a new character and pick other faction without sacrificing anything you’ve worked for.