They can sell it above 100 tc, because it is sold out in the Torishop which increases its value.
A little exampel :
Lets say you bought 4 Baseball trading cards for 10€ (2,5 each) and the ywould be sold out after that.
When people want to buy them now they can only buy them from somebody who own them like you.
So you want to make profit out of it and you sell every card for 10€ and the people who buy them will pay that price cause the got now other ways of getting this card.
That's a one solution, but some people also give some items ridiculous prices hoping that someone would be stupid enough to buy items for stupid prices or accidently buy the item since as far as i'm aware you don't get any refunds for accidental purchases.