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ICv2 has an intersting article up about the differences in the Diamond estimates they calculate and the worldwide preorders provided by Image Comics for the 2013-03-15 releases:

http://www.icv2.com/articles/news/25506.html:

Image Comics provided us with worldwide (including Diamond UK) pre-order numbers for 15 March releases, so we could compare them to the ICv2 estimates for sales from U.S. Diamond locations in March, drawn from our over-all estimates of Diamond U.S. sales to comic stores (see "Top 300 Comics Actual--March 2013").  So the differences between the numbers reflect two things:  the pre-order numbers include Diamond UK numbers, and the ICv2 estimates do not; and the ICv2 numbers include reorders, but the pre-orders do not.  One other key difference on East of West #1 is that the pre-order numbers include over-ship numbers; the ICv2 numbers include a Diamond reserve for returns.

Typically, we've seen the Diamond UK numbers amount to about 10% of the total Diamond sales in the past, but that percentage can vary wildly depending on the title.

The Diamond data equates to as low as 82% for East of West #1 (with Diamond reducing the numbers for potential return of the overshipped copies) to as high as 99.62% on Peter Panzerfaust #10 with only 42 additional copies in the Image worldwide data. The average is 93.03% which is a little high than I would have expected.

As I've always stated, the Diamond data isn't the complete picture but it is the overwhelming majority and represents the core comic book reading audience.

Being from the UK I presumed automatically; of course the overseas amount makes up such a small percentage because 'brick-and-mortar' stores are much few-and-further between and going to a physical non-comic book store and buying a comic is much less common. It was then when searching on diamond's site for the first time I found that their numbers include internet stores etc, would this include all official ebay stores and casual ebay individual sellers? I have no idea if these sellers source their comics the same way having never attempted to sell comics.

As a kid I was bought Roy of the Rovers and TMNT from a local cornershop out of a choice of quite a few US titles, but now outside of a few Simpsons and Barbie comics aimed at a younger market you'd find nothing in a local newsagent, and a physical comic book store would be much further away, and a smaller presence than in the States. For quite a while the internet store, or seller on ebay, I would think has been the common route for buying comics in the UK and therefore the extent to which these sellers are reflected in numbers would play a great part in the degree to which overseas sales as a whole are reflected.

Of course this is a purely personal anecdotal piece of evidence and it may be other UK buyers are able to contradict this.