> The article makes it sound like there's collusion or price-fixing between galleries, but there's no evidence of that.

I would say that a gallery bidding on an auction because they have an interest in increasing the transaction price of pieces being sold by an art dealer connected to them is a textbook example of collusion (as opposed to bidding because they want to own the piece).

As usual free market fundamentalists will disagree that this is wrong in any way.