So many UK sellers state that they are prepared to sell to 'UK Only', and this seems surprising to me. Why cut your listings off from such a potentially substantial market? Especially as it takes little more effort to send an item overseas than it does inland.
Whether you're sending your parcel to Hawaii, Hong Kong or Hull, it makes very little difference, just as long as you charge your buyer the correct postage (having a set of small postal scales at home are really handy for this, and postal rate charts are available free of charge from the Post Office, and on the Royal Mail web site). You will need a Customs document for all Small Packets and other packages containing goods up to the value of £270 to destinations outside the EU and these specific EU destinations: Andorra, Canary Islands, Gibraltar, San Marino and Vatican City State.
Airmail Stickers and Customs Declaration label (or CN22) are both are available free of charge in sheets/rolls from the Post Office and the Royal Mail web site.
Some people worry about the Customs label, but they're really self-explanatory, and once you've completed a couple of these you soon become pretty adept at filling them out, and instructions are included on the back of the label with examples on how best to describe your item.
Just remember to tick the relevant box to say what type of item you are sending, describe the contents as clearly as possible, and make sure you include the £ sign or write GBP with the value amount. Sign it, date it, and it's done!
Next, stick both the Customs Label and the Airmail Sticker on the parcel, preferably on the top left corner of the side where the recipients address appears. It is also important to include your return address on the back of the parcel.
If you want to offer your international buyers Postal Insurance you can, it works in very much the same way as Recorded Delivery and Special Delivery do for UK addresses, and all you need to do is obtain an International Signed For label from the Post Office. Details of charges and compensation levels etc can be found on the Royal Mail web site.
And that's it! Easy eh? Hope this has helped :-)