With DC’s release of a total number for sales on Justice League #1 through mid-December (see “’Justice League’ #1 Over 360K”), it’s possible to get an idea of the portion of sales not accounted for by the comic store market serviced by Diamond Comic Distributors U.S. (which services comic stores in North America and other parts of the world not serviced by Diamond UK). 
We took a look at the sales on Justice League #1 through Diamond U.S. between release and the end of the year. Our monthly sales estimates for August through October (see “Top 300 Comics—August 2011,” “Top 300 Comics—September 2011,” and “Top 300 Comics—October 2011”) total around 256,000 copies. Diamond continued to sell Justice League #1 after October (in fact, a fifth printing shipped in January), but sales apparently did not meet the threshold necessary to make the Top 300 titles listing. The bottom of the chart for November and December was around 4000 and 3000 copies, respectively, so if we add that number to the total for August through October (assuming without evidence that Justice League #1 just missed the bottom of the list, in part to account for sales of the digital combo packs), we get 263,000 as the number of copies of Justice League #1 sold by Diamond North America in 2011. 
The difference between the roughly 263,000 copies we’re estimating that Diamond North America sold of Justice League #1 and the 360,000 in total copies sold reported by DC, some 97,000 copies, or 27% of the total, comes from two main sources: international and returnable sales. 
In general, we’ve heard that Diamond UK sales, which are not reflected in the Top 300 monthly sales estimates,  account for around 10% of Diamond’s total sales, although that fluctuates widely depending on the title. 
So if we assume around 295,000 copies sold by Diamond worldwide, that leaves 65,000 copies, or around 18% of the total, through other channels, which include what’s left of the newsstand market and returnable sales in bookstores through Ingram Periodicals.   That’s probably a higher percentage than what’s typical for returnable sales, but we do know that there was some channel migration by consumers looking for copies when comic stores were briefly sold out shortly after the book released (see “Two Batbooks to $3.99”). 
The bottom line? Justice League #1 was a top seller in returnable channels as well as in comic stores. Comics selling 100,000 copies in comic stores (and there are precious few of those these days) are probably selling 10,000 to 20,000 copies through returnable channels at best. If Justice League #1 did, indeed, sell 65,000 copies through those channels, it was one of the bestselling comics in returnable channels in recent memory.