Gold Value Guide: Item Market vs Black Market

To avoid confusion, I would currently use Nerve Tonic as the main reference, since Energy Drink prices dropped.

Nerve Tonic: 15 gold, sells for around 200k
So 1 gold is worth around 13.3k.

That means the Black Market value is roughly:

Green: 3 gold = around 40k
Blue: 10 gold = around 133k
Purple: 30 gold = around 400k

So Item Market is only clearly better if the item sells above these values, after fees.

Green: sell on Item Market above 40k
Blue: sell on Item Market above 133k
Purple: sell on Item Market above 400k

Below those values, taking the gold from the Black Market is probably better.