Addressing Serious Hacking Issues

There are ways to generate fake packet signatures and circumvent checks like you suggested. Easily, in fact.

Not in a useful way. Even if the packet is signed right, if the server refuses to swap the player's item/meso for the requested item, there's nothing they can do about it. That's the kind of check I'm talking about. The only way to get around it then is to find a loophole in the server's limits, and that's trickier. (like say it's whitelisted maple items to trade for silver coins, then unless another item ID accidentally gets on the list, there's no way to sell arrows for coins)