I'm not sure I understand your question. You DO want your web pages to at least be on the same general topic. For example, web hosting and vacation packages dont generally go under the same domain. On my dart site (CorkitUp.com) I sell dart supplies only... and all of those are through the same merchant (though I am not opposed to offering products through various merchants - there just werent a lot of options in this particular niche).
You can have more general sites. For example something like "sports.com" and then tons of more specific categories (basketball, baseball, golf, etc) and within each category links to even more specific pages (baseball memorabilia, baseball tips, baseball stats, etc).
I hope that helps...