I will be the one to break away from the hate-train this thread has become. As someone who sells a fair amount of stuff (usually on eBay, not skiing related), it is important to be up front about the condition of the item you are selling. When condition is not specified, it is assumed that it is not important....that is is obviously in good condition.
What you received is NOT in good condition. I don't know if they are worth $90 or not, but it is important for people to stand behind what they sell. To sell an obviously used/abused item, and not be up-front about it is no different than lying/stealing. Stand behind your product, make the customer happy, and refund his money. If I was the seller I would first offer a partial refund ($25) and offer to let you keep the pants. If this was not acceptable, a full return and refund would be in order. I do agree that you should perhaps pay the return shipping, as you did not expressly ask about the condition. This was your fault, and you learned from it, but the fault lies with the seller. Seller, time to man-up and do the right thing. If the seller does not do so, he is clearly not a man, does not hold himself accountable for his actions, and you should obviously contact his parents as they are still responsible for their son's behavior.