if you're buying from a company you're not familiar with then make sure they are displaying a telephone number on their website.
Try the number and make sure they exist.
If they have an address and post code. Check it out on BT's directory enquiries website to make sure it is genuine.
When you are entering personal details, make sure there's a little padlock icon on the bottom right of the screen.
Don't deal with anyone in Nigeria. They have the largest number of internet fraudsters in the world.
I would start by speaking to your local police, and possibly a chat with you local councillor or mp may help. They should at least know where to go. If it is credit card/bank fraud, contact the card issuer or bank.
The first line of defence is YOU. Do not deal with any person or company you do not know and trust. If it seems too good to be true, it probably is. I have conducted many transactions online, following these simple rules.
Seriously though the main catches currently are the variations on the Nigerian scams, where helping someone to get Millions out of a country you end up paying "expenses" up front.
Also "phishing" scams are where you recieve an email from a bank asking to confirm your account details. A link is always on this mail taking you to a web page that is NOT your bank, no matter how authentic it looks. When you "confirm" your details they log on to your account & transfer out your money
I bought something off Ebay for 1300 pounds and never got it. Ebay are useless and the npolice aren't interested. Could go to a solicitor but that wpould cost even more and even then I doubt if I could recover any cash.