I've both bought and sold internationally on
ebay. For an overseas purchaser, I'll dig out a couple of shipping methods/carriers/prices and let them choose...all via email before I ship. Once I'm paid for the item and shipping, off it goes. Note that if I had listed an item as free shipping, the free part is limited to US only. Although I sometimes deduct something from the international shipping price I quote them comparable to what I think the free shipping would have been stateside.
Except for the minor paperwork addition, selling to someone overseas is not a hassle. However, my overseas sales were limited to the under $100 items. Higher than $100 value, I'll put a US-only clause in the listing. The reason is two-fold. One, if I get ripped off somehow, my recourse for an overseas sale is likely so problematic that I'll simply take the loss. Number two is declared customs valuation. I always put the sale amount on the declaration. Some buyers want a lowball figure to avoid import duties, but for an item less than $100, it's a small enough duties charge that they won't have a problem.
Similarly, I don't have a problem buying from overseas shippers. Some years back, I bought a good number of computer hardware components from Canada and never had a problem. These days, I've had good luck buying genuine Epson printer ink from China...the prices are unbeatable. Again, I limit my overseas purchases to perhaps $50-60, in the event I never get it. The one exception was a used Canon 80-200 f2.8L lens I bought several years ago from what seems to be a Japanese camera store with an exceptionally good
ebay rating. It arrived two days later!
I guess it's a matter of choice for some sellers to refuse overseas sales. Too much risk in their minds? Perhaps they're unwilling to add the extra 2-3 minutes of work to get it shipped? Who knows.
I guess my biggest problem buying from overseas is transit time. I've had ink from China consistently arrive in less than a week based on postmark. However, at the moment, it's going on 2 weeks since I bought an item on Amazon that I didn't realize was coming from Great Britain. Still waiting for that one.