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A big part of the marketing advantage that publishers have is that they have access to this kind of information. This is a problem even for authors going the traditional route, because they are expected to provide comparable titles that has sold well as part of their query package, and it is hard to get the sales information you need to do that.
I believe there is a database on book sales that you can buy access to, but it isn't cheap.
This is one of the misconceptions about self-publishing. People are told that they have to do their own marketing. But by this people usually mean their own publicity. But the real problem with marketing is to get the data you need for decision making, and that is very hard to get when you are on the outside looking in.

A big part of the marketing advantage that publishers have is that they have access to this kind of information. This is a problem even for authors going the traditional route, because they are expected to provide comparable titles that have sold well as part of their query package, and it is hard to get the sales information you need to do that.
I believe there is a database on book sales that you can buy access to, but it isn't cheap.
This is one of the misconceptions about self-publishing. People are told that they have to do their own marketing. But by this people usually mean their own publicity. But the real problem with marketing is to get the data you need for decision making, and that is very hard to get when you are on the outside looking in.