If you do say, 100 araxxi's and you complete a spider leg. You gained the profit of the spider leg's cost if you sell it on the GE right away. Disregard any cost of supplies. So right away you now have +143m but you want to buy a Noxious staff or bow. Let's assume you are short 3m of the +143m you just made from the leg and you have at least 3m previously before you sold the leg.

You just bought a noxious weapon for only 3m. At the end of the day, you profited +140m.

Now let's say you held onto the money for a few days and the cost of one of those two weapons is now only 141m. Gee, now you have 2m leftover. You retain the profit of the cost of the weapon you spent + the leftover money. +143m with 2m unspent