Is it permissible not to buy a piece of clothing after trying it on and leaving the store angry because of the seller's mockery, when a similar piece is available at a much lower price?

What transpired between you was merely a promise to purchase. Fulfilling a promise is commendable and not obligatory, unless the promisee incurs a cost as a result of the promise, which is what the Islamic Fiqh Academy has decided. It appears that the seller did not suffer any harm. If you wish, you may purchase the plot of land in fulfillment of your promise, which is preferable. If you do not wish to, you are not sinful.