This is the first guide that I have written, however I've seen a few guides trying to discourage buyers from buying goods from American buyers and wanted to offer advice.
I've probably spent several thousand pounds buying clothing through eBay from buyers in the United States and have found every single seller that I have come across to be extremely helpful & informative.
Of course, there is always the chance that customs may look at your goods and charge you the import VAT which can vary as there are normally other costs on top, however I have only ever been asked to pay this once and it was a minimal amount. I'd recommend that you bear that in mind when ordering.
One of my purchases was 3 pairs of Nike Air Shox trainers - that I bought for a combined amount (including shipping) of about £80. 1 pair in the UK retails at £100 (or they did at the time) and as I wear a size 15 that are impossible to find in the UK I was delighted. No charges from the customs people - I think the trainers arrived within 4 working days!
I'd recommend Express Shipping, or even pay a bit extra for a courier and if the seller is decent and you are only buying 1 item, you can ask them to put the value on it - I don't think customs look at anything below about $20 so most will arrive without any trouble whatsoever.
Also, don't be put off by sellers that say they ship to the US only, drop them an e-mail to ask if they would ship to the UK as most are very happy to do so. Again, most of my items were bought from sellers who had only previously shipped to the US only.
....and try to always remember to leave some feedback as it helps build the international community! Happy Trading!