Hi Emmanuel,
Being a Spanish resident, I've experienced the same kind of problems and frustration over "US only" as yourself, so I can understand you perfectly. These are some of the rational reasons for it, IMHO:
1) The US is such a large and powerful country and thus such a large market, that any seller can confidently expect to get many more local bids than foreign ones. The foreign 'share' is so small next to the local one that just plainly ignoring it isn't likely to affect business noticeably.
2) That being so, most sellers, specially those that do not run a business, don't feel comfortable dealing with foreign bidders, which probably aren't native English speakers and so there's the possibility of misunderstanding ("mint", "NIB", "no COD", etc), and which perhaps could result in
excessive delays or charges to get the payment and/or send
the items. There's always the possibility of a scam lurking out there.
3) Finally, sending a package to an US address is very easy and there's a choice of ways to do it, many of them requiring very, very little effort on the seller. On the other hand, sending a package abroad requires a cumbersome, delicate packaging, with plenty of protective material if the calculator inside is going to stand the rough treatment of a 6,000-mile trip, and that costs money and, specially, a lot of seller's time. Also, the seller is forced to fill and sign a number of forms for the Customs, declaring the value of the item, and a number of other formalities, which many sellers consider a real, big nuisance and yet more time spent.
There's also the fact that many foreign postal services are not as optimal as they should be, and tremendous delays and even lost packages are extremely common, resulting in a lot of inconvenience for the seller. Not to mention when the item arrives broken and it was in perfect working order when it left the seller's hands.
Taking all of these into account, it's easier to understand why many sellers will opt instead to restrict their sales to the big, familiar, comfy local US market. They might occasionally get a slightly smaller final bid for their items, but they'll save a lot of troubles, headaches, effort and time on the long run. Just put yourself in their place.
Best regards from V.