sellers can get together and try to discuss prices, to either lower it or push it up from an original price.
If sellers are coordinated and organized enough to set prices. Its no longer a market arrangement. It more closely resembles a monopoly. Which will typically lead to price gouging and reduced product quality and reliability.
Something resembling a monopoly on the consumer side, will usually resemble a boycott. Or the so called "retail movement" in stocks.
Its much easier for sellers to organize than buyers. They have the networking, communication channels and infrastructure.
Buyers tend to be more unreliable with less throughput on promises.
Each has their pros and cons. Which the majority of people do not understand. Considering most seem to not comprehend how the price of healthcare has become overinflated around the world.