Why do people on eBay not ship overseas?

I do not live overseas, but I was wondering if there was a particular reason why most eBay sellers, or other internet vendors, do not ship their products to foreign countries. I know some they do, like Canada, the U.K. or Australia (but more often not Australia). But I am also curious about places like the former USSR and African countries.

Is it just the hassle of figuring out the shipping for each individual country? Or are there other barriers to this that cause many people to avoid servicing these regions.