I sell only small and inexpensive items here, so sending it overseas makes no sense because shipping become a major portion. If I were to sell more valuable items then custom becomes a concern. If the item gets stuck in custom, I have no control over it. If a broker gets involved, it gets really expensive. Again, I have no control over that either.
Long time ago, I had something shipped from Japan. The package simply disappeared and I had absolutely no recourse. Once, I shipped to Japan, and again, it went somewhere else. (yes, I speak both English and Japanese) Once something goes wrong and the item crosses border, what should be simple can get really complex and impossible.
If the other party was either in Canada or UK and we can reach a mutual understanding and I can trust the person, and the value is small, then I am willing to consider exceptions - but as a general rule, for me, it's US only.
I'm also very careful with whom I'd deal with, selling and buying but that's not what the OP asked, so I won't elaborate. There's no way I would make exceptions on eBay. (I no longer use ebay. Really bad experience)