Ask small or unknown companies for references or testimonials, and if you have any suspicions, see whether Trading Standards has heard of them.
By using a credit card to place a deposit, you're protected by the Consumer Credit Act. This means you can enlist the credit company's help to get your money back if anything goes wrong.
Don't hand over details of your debit card unless you're really sure of the company you're buying from.
Only place deposits on websites if they use encryption technology to protect your personal details.