Price iscrimination exists when sales of identical goods or services are transacted at different prices from the same provider. In a theoretical market with perfect information, no transaction costs or prohibition on secondary exchange (or re-selling) to prevent arbitrage, price discrimination can be a feature only of monopoly markets. Otherwise, the moment the seller tries to sell the same good at different prices, the buyer at the lower price can arbitrage by selling to the consumer buying at the higher price but with a tiny discount. However, market frictions in oligopolies such as the airlines and even in fully competitive retail or industrial markets allow for a limited degree of differential pricing to different consumers. Price discrimination also occurs when it costs more to supply one customer than it does another, and yet the supplier charges both the same price.