Vendors restock every long rest, including gold. Even if all you do is sell junk to vendors with maximum affinity, you'll have over 40k gold by the time you're in the mostly through the underdark, and that's WITHOUT farming crystals to sell.
If that still isn't enough, you can just steal from vendors to loot them of their gold at no risk and no repercussions, endlessly, resting in between emptying them out to create an infinite amount of gold.
Also, don't kill vendors, it's wasteful ^.^
Edit: Even if you get caught stealing, just bribe them (for less than you stole), and then pay them some money back free to return their affinity to maximum... then steal it again. There's simply no consequence to this and no fail state.
