Clearly the goal here is not to simulate an ecosystem where agents, buyers and sellers happily live together.
However, the idea here is to organize your object model exactly as you would organize a group of employees. Each of these employees has a specific job and a specific set of responsibilities - ideally only one. You can describe the solution to your problem as the result of their interaction. These employees are actor objects.
To interact efficiently, they need to exchange information, in the form of data objects. These data objects don't do anything except hold a piece of information - exactly as you would have two employees exchanging notes or emails.
Most of the time these data objects correspond to the language of your problem domain. The actor objects however may have nothing to do with any "real world" activity, depending on the job you want them to do in the process. So yes, the object model diverge from reality over time because we introduce new actors with specific roles that have no "real world" equivalent. But that's just fine.
The important thing is to keep seeing the distribution of responsibilities among your objects and their interaction as the work of a team of independent experts, not as "things doing stuff".
However, the idea here is to organize your object model exactly as you would organize a group of employees. Each of these employees has a specific job and a specific set of responsibilities - ideally only one. You can describe the solution to your problem as the result of their interaction. These employees are actor objects.
To interact efficiently, they need to exchange information, in the form of data objects. These data objects don't do anything except hold a piece of information - exactly as you would have two employees exchanging notes or emails.
Most of the time these data objects correspond to the language of your problem domain. The actor objects however may have nothing to do with any "real world" activity, depending on the job you want them to do in the process. So yes, the object model diverge from reality over time because we introduce new actors with specific roles that have no "real world" equivalent. But that's just fine.
The important thing is to keep seeing the distribution of responsibilities among your objects and their interaction as the work of a team of independent experts, not as "things doing stuff".