Christian,
I know that I can use UPS and Fedex (DHL closed up shop here in the US a few years ago due to lack of business), but it's just an extra hassle. I have to go to the post office every day anyway, so that isn't a problem for me. Shipping overseas requires extra forms and extra costs. And while some countries have little or no issues, some countries are just to much hassle to deal with, and I don't have the time or inclination to figure out which ones are good and which ones are not.
If I was selling things on eBay as a business, I would sell to overseas customers, but selling on eBay is something I do to get rid of extra stuff I don't need. Most of the time the money doesn't even leave my PayPal account before I have spent it on more kits I don't need.
I don't buy stuff from overseas either, for all the same reasons.