I have 100’s of pounds of semi sorted Lego, mostly by colour.
Would it be beneficial to sort by colour and part to sell in bulk? Or sell a bulk lot of one colour but various bricks?
It (obviously) depends on what people want to buy.
If someone wants to build something, and doesn't totally care what colors are used, then they'll want to buy categorized by shape. It's so much easier to find a red 2x4 in a bin of 2x4s than to find a red 2x4 in a bin of red pieces.
But if someone wants to build, say, Big Bird, then they'll just want yellow, and will want to buy categorized by color.
I think the former is much more common.
See also: Organizing your LEGO Bricks