We've all seen it before, a bargain in the USA on Ebay.com, seller will NOT ship internationally, what course of action is available to a buyer? in the first instance, a good communication through the 'Ask the seller a question' facility, setting out your sincerity as a buyer, inviting them to look at your feedback history, & pointing out that the United Kingdom, or your country has a secure postal service, sometimes this will work, although I have encountered a seller who's 'Ask the seller a question' feature was blocked to overseas members, perhaps an internet search may reveal a business sellers phone number, in which case, a phone call may also assist in securing the item.
It's very important to do ones homework & research this thoroughly, shipping costs can be prohibitive, plus there may be customs charges as well, it's very much a question of the item being worth the value of the purchase plus the expenses, or as to it's rarity value, a good example is a spare part for a classic vehicle which is no longer manufactured, this is especially so of Japanese made motorcycles.
The final option available to a buyer is the use of a re-shipper service, their costs can be quite expensive & must be taken into account. they would bid or buy the item on your behalf, which is then shipped to a warehouse facility for onward trans shipment overseas, it's then a case of good luck & happy shopping.