If you’re thinking about purchasing comic books for possible financial gain your entire mindset and strategy must change versus the run of the mill collector. More than ever you are going to be concerned about the law of supply and demand. With this in mind, your tools must be carefully selected. An excellent tool for actual distribution numbers is Comichron. This resource was put together by John Jackson Miller and is a core part of the Standard Catalog of Comics.
Comichron has sales data going back to 1996, so it is most valuable for new comics. You can read more about how the data is derived here.