The respectable case is traders who transfer good between supplier and buyer because there is no smooth transfer path or when there is significant risk in communicating directly with the selling, such as stocks, grocery stores, and realtors. The non-respectable case is when the trader exists purely to hold onto then profit on the product. This includes domain squatters, ticket scalpers, and those people who buy things from one thrift store to sell back to another thrift store at a higher price. It's the same reason why patent trolls are so disliked.

Without ticket scalpers I couldn't decide to go to a sold out show or ballgame at the last minute. They provide a valuable service to me.