What is the name for a market where everyone is basically selling the same thing?
Such markets are said to be homogeneous goods markets or commodities markets. And the goods traded within them are sometimes known as commodities. From Wikipedia:


  "In economics, a commodity is an economic good or service that has full or substantial fungibility: that is, the market treats instances of the good as equivalent or nearly so with no regard to who produced them. Most commodities are raw materials, basic resources, agricultural, or mining products, such as iron ore, sugar, or grains like rice and wheat. Commodities can also be mass-produced unspecialized products such as chemicals and computer memory."