I've sold lots of stuff overseas, once to the US, many times to Spain and once to France. Use the international signed for service, which will give you proof of receipt, but unless you use a courier (UPS, Fed Ex etc) you can't track a package once it's left the UK. The postage costs can be on the high side but that's just something to manage with the auction listing. To illustrate, I once sold an old hard tail frame only to a guy in france. The postage was £120 insured and with proof of receipt.
The big news is that you can significantly raise the final value of the auction by posting overseas especially to places where parts are expensive new (I think Spain must fall into that category) and/or where the pound is weak against the local currency.
I usually put the highest price I can reasonably think of for international postage costs. I then add a note in the auction listing to say that this fee must be paid on the close of the auction and that I will refund any difference after I've packaged it up and got a fixed price for postage. I've done this several times and it has worked well, with people being very appreciative of a the refund.