You can find on our site under "License" all you need to know, in particular it is stated "for personal use".
Our main target is internet marketers category but would like to expand it to bloggers as well. Usually our clients buy blog themes and are not satisfied with header images. We give them alternatives.

Our headers are altered/photo effects manipulated/improved. The buyer gets a Personal License which states conditions of use. We cannot control ( and it's not our wish) what the client decides to do with the design.

Our extended license allows us to use image(s) as original in a limited number ( 10000)

"If that is not allowed, who on earth is going to buy them"
You may want to reformulate this way of speaking as we do business in a very polite manner. To answer: if a client buys a header he is entitled to use it as he pleases. If he wants to sell his site and use the header image then he must purchase the proper license from the stock image site

"how many of these headers have been sold here?"
Inappropriate question