Why are spectre skill gems flying off the shelf?

My husband has been playing the merchant lately, filling his shop with potions and skill gems and whites, just to see what will bring people in to look at the rest of his stock.

He's found something strange. He's listed about a dozen level 18 Spectre skill gems at 1ex a piece, and at least once a day, a minion player walks in and buys the entire stock.

That makes no sense. Raw skill gems are a penny a pound, you could just cut them yourself.

So the next day he bumps the price up to 2x. Another dozen gone in one visit. Another minion master.

Next day, 3x. All bought in one visit by a minion master.

Someone make this make sense, I'm begging you. They don't have a failure rate, they don't come with rare abilities, they can't be resold after capture, and you're not going to use more than two or three different summons at most.

What am I missing?