I agree with you on the fact that supply and demand is a thing, but what happens when supply is plentiful and demand is high?
People are still gonna charge the same generally. Look at (Green) Midori, he had like 5000+ materials, yet chose to sell at 5:1 because he could, and would "justify" it by saying that "It's just supply and demand..". Most people don't quite wanna hunt their own materials, despite saying the rate is 10:1 (myself included on both parts, but ill hunt my own usually) so they settle and buy for 5:1. Now, he could've sold at 10:1 and still probably made the same profit, albeit slightly slower. Then again maybe faster since it's a better rate for selling.
I'm not saying people SHOULD charge the same, but it'll most likely happen. Since most people prefer profit over selling for cheap and (sometimes) helping someone out.