If you were selling it on the RPF I'd suggest having a short dialog with the interested person and make sure they know what laws apply for their country. Sweden, for example, would be absolutely no problem as long as you aren't trying to send a REAL, deactivated or blank-firing firearm. The only other issues you might have would be stilettos, butterfly knives (don't ask my why), brass knuckles and high-power stuff like crossbows. Replicas and airsofts are no problem to import here.
However, since we're talking ebay... and frankly I'm more afraid of taking a chance with getting scammed through there these days... I'd be a little more careful. With ebay's/Paypal's current way of handling things (i.e. "screw the seller") I'd be a little hesitant to send to England or Australia without having some proof they'd get through. The entire UK is a gamble these days unless the buyer can prove he's going to put it in a museum or something. When shipping to Australia I think the receiver has to arrange an import-declaration with their local police office or something. Germany should be ok since you can easily buy MR blasters and stuff there, but I don't have any firsthand experience. Hong Kong is one of the worlds #1 places to get replica guns like airsofts, but I don't know what they say about importing.
You could always ask for payment via money order if you're worried- that way the worst that could happen to you is a bad feedback. Or you could demand they pay for EMS/Global Express shipping with insurance, tracking and delivery confirmation to cover yourself. (EMS is pretty reliable and available at the USPS.)
Bottom line: the biggest problem these days as a seller is sending to complete unknowns while accepting Paypal, especially CC payments, and running the risk of getting a chargeback against you. (But that, on the other hand, can happen in the US as well.)
If you want... visit the USPS site. I wouldn't be surprised if they listed information on import restrictions for most countries. (The Swedish PO site does so at any rate.)
