For those who do not rely on heter mechira (selling) for bal yera'eh bal yimatzeh (the prohibition to own leaven on Pesach) for chametz gamur (certain leaven), do they generally sell flour or get rid of it as well?
If water hasn't touched the grain, the it certainly is completely Kosher for Pesach.
The Talmud (Pesachim 40b) tells us that wheat (but not barley) can be tempered with water prior to milling without fear of Chametz. That said, the longstanding custom from the Geonim (recorded in Rambam Chametz 5:7, Tur OC 453) is not to do so for flour for Pesach, lest one not temper and immediately grind efficiently and the wheat will ferment and become Chametz. The Mishna Berura (453:27) rules that though one may not eat flour from tempered grain, one may own it on Pesach.
Regarding flour that you find in modern stores, it's mostly a question of how they actually do it. Do they leave it in water for a while? Do they grind it right away? Perhaps modern plants can clean and grind the grains more efficiently with even less water than the Geonim would prohibit? See this link for some modern opinions about flour from stores ranging from burn it, to sell it, to just own it (but don't eat it).