Make sure that you're familiar with the tax situation in any country that you submit to, including the forms you have to file in your home country when you receive monies from out of the country.
If you're American, but sell a book in England, then you've not only got to file a form with the IRS explaining that you're getting money from sales in England, but you've also got to file the proper forms for the English version of the IRS stating that you're not an English citizen so that they won't tax your income from sales as if you were.
You pretty much have to do the same in every country you sell through. (Which is one reason an agent can come in handy. They know all of this stuff, and send you the pertinent forms, so you don't have to keep track of it all by yourself.)