Until I’ve got 500K iron - any double yellow weapons, off-hands, shields and accessories, since armour doesn’t sell for that well, along with any and all greens. Once >500K, greens (bar single affix MI’s) of the above + double yellow accessories. Usually nets me a decent income, but for rouge likes I usually only pick up single hand weapons, since usually blues take up most of my inventory.
And once I hit 5 million iron bits, I only pick up green jewellery to save time porting back to sell stuff, along with any MI’s that look like they’ll be useful or are needed for crafting. As for why I aim to have such a massive stash of iron - means I never need worry about running out if I gamble at the celestial smiths, have a top tier relic to craft or other crafting fun.