Assume resellers will buy poison from you at half the market value, so 100gp for Serpent Venom, 150gp for Carrion Crawler Mucus, 600gp for Wyvern Poison, and 1000gp for Purple Worm Poison. The market value of a basic poison is 100gp, and pretty much every poison you can harvest is better than it. However, DMs are totally within their rights to change the market value of things or to make them difficult to sell since most honest people don’t have a use for poison.