A local market's price answers to demand and supply in one place
Some goods and services cannot be moved far. A flat to rent in Leeds is no use to someone who works in Bristol, and a taxi in one town cannot pick up passengers in another. Each of these is a local market, and its price is set by demand and supply in that place alone. When a large employer opens in a town, rents rise in that town and the area around it, not across the country. The higher rent rations the flats that exist, and it draws landlords and builders to that town rather than to one where rents have not moved.
Every buyer of oil or wheat pays one world price
Crude oil and wheat are cheap to ship compared with their value, so buyers and sellers across the world trade in one global market. Every buyer pays the same world price, give or take the cost of transport, and no single buyer controls it: a chemical company uses oil as a key input, but has no control over the world market price for crude oil. When a drought cuts the harvest in one large wheat-exporting country, world supply falls and the world price rises for every buyer on every continent.
Before the droughtAt the world price P1, buyers across the world want Q1 and growers supply Q1.