You can call your business whatever you like - on your tax return you wil be CWA trading as....
Limited company names are registered but trading names aren't. Do a quick search on google and ebay to make sure your chosen name isn't already used.
Check the name you want isn't being used by a limited company here:
http://wck2.companieshouse.gov.uk/3a6a19bfb174 1abf318e9972b660ec54/wcframe?name=accessCompan yInfo
You will have to fill out a tax return each year and pay income tax if you have made enough money. Even if you make a loss you have to fill out those forms. Don't forget that income from other sources, such as paid employment, is also taken into account as far as your tax free personal allowance is concerned.
You can hold down a full time job, a part time job and have as many businesses as you like. If you do not have paid employment you will have to pay NI contributions yourself, �2.30 per week at Class 2, and Class 4 if you make over �5435.
From the very start keep records of every penny you spend on your business, and every penny you earn. Keep the receipts.
Make sure you understand the law. If you are trading on eBay you will be subject to the Distance Selling Regulations if you sell on the Buy It Now formats, but not if you sell in the auction format. Make sure you fully understand your obligations under the DSR.
You will also be subject to the Sale of Goods Act so make sure your goods are as described, fit for the purpose and of reasonable quality.
You must tell the Inland Revenue of your self employed status within three months of the start up, or you will be fined.
Keep on top of your record keeping.