Internet auction sites

Questions:

• Have you got any tips for purchasing goods online and do they only apply when bought in the UK?
• I am the victim of a crime that has happened abroad, what do I do?
• I have bought an item from eBay /or other auction site, sent the money and the item has not arrived...
• I want to buy a car that has been advertised on the internet, how can I tell whether or not it is genuine?