Although the issue is getting out of hand, and gilsellers in sky is deadly, gilsellers may also realize that sky doesn't offer everything they want. While gods to yield some great crafting materials (probably the main reason they are there), they also yeild a lot of Rare/Ex items. This makes them more proficient as well as a side-effect.
I won't say I know everything about sky. Infact, I don't even have access, so that limits my complete knowledge. But as we all know with gilsellers, without a market, they make no profit. Hell, if only 1 person buys gil, you only have 5 people sell gil. The less amount of people that buy gil, the less amount of people that will sell it. Thus, they also lose people to fight gods and NMs. 1 person cannot solo powerful NMs.
So, and I know it's been proposed many times, 'adopt a newb'. Anyone coming into the game should know that you don't need great items till endgame. Hell, if it comes down to it, say that having great items early on is bad for them, even if it is mostly a lie. If they see someone that's Rank 10 and has lots of cool equipment, they will probably have a level of respect for you. I know it isn't the only solution, but spreading the hate against gilsellers and saying buying gil will only come back to stab you in the back endgame (which is true) will help get rid of them faster then you may believe.
Just my 2 gil.