I thought that cartel implies that sellers collude to artificially raise prices and limit competition. If this is the natural result of a free market, it seems that it is therefore not a cartel, just that given competitor price information, when the seller reacts first the low-priced sellers are likely to raise their prices (get more profit when the buyers come) rather than the high-priced sellers lowering their prices (gain customers from other sellers) which happens when buyers react first.

It is an interesting observation/model but calling it a cartel is a little bit over the top. The good/service reaches its market price regardless of whether it gets there from lower or higher prices.