Responses
It's literally the concept of trading. You can't just get their skills without having to pay equal value as it may result to unfairness to either the trader or to his supposed customer. If he could just get their useless skills without permission, then they wouldn't have called his skill "Trading".
My guess optimistic guess is that the sister wanted to collect all the skills she stole from plundering and give it to her brother. Since there's a mention that it'd be nice if the brother could have one of her skills. But meh who knows.
I really don't get why the mc pays a lot for useless skills when he could have offer the service of gimme your useless skills that you don't want and couldn't be removed, but you have to pay me for a price in exchange. But yeah that's the plot, its not like I could change it, so I'll go along with it.
6.5/10