In the UK, you have "added value" to a bunch of products and provided a service. You would be expected to pay the government VAT (Value Added Tax) which you would pass on to the customer. If you are exporting, you don't have to charge them, usually when you ship it, you can use the courier company to help, you mark the package with what it is and the value and the customs and excise for that country will apply the tax. Naturally a lot of people try and circumvent this, for example by mis-labelling goods.
Your best bet for getting advice on this from people who do it all the time are the eBay forums.
Rich