Safe shopping

Look at several sites to check prices and be aware of any hidden costs, e.g. VAT, delivery.
Shop with well-known companies or following recommendations.
Research the company if you haven't heard of it before, look in papers and magazines.
Know who you're dealing with - if in doubt look up firms' address and phone number and call to double check details.
Have a geographical address, be wary of just a PO Box No.
Find out refund and return policies before you place an order.
Check how long delivery will take. If the supplier is made aware of a vital delivery deadline, failure means you can cancel the order and have a possible claim for compensation.
Phone the company if there's anything you're unsure of.
Be very careful of company policies if purchasing from overseas. You are not necessarily protected under UK law and it can be very difficult to pursue a dispute.
Buy with a credit card - it gives more security than a debit card as the credit card issuer is also liable for misrepresentation and breach of contract, so if goods aren't delivered you can pursue a claim with your credit card company.
Check credit cards details if buying from overseas - check foreign currency.
Keep records of all ads, confirmation e-mails etc.

The Law states goods must be:

as described on the web site. According to the Advertising Standards Authority (ASA) descriptions must be "legal, decent, honest and truthful".
of satisfactory quality, free from defects.
fit for their purposes.