Conmen have tricked Britons out of at least £350m in the past three years, according to government figures.
Its difficult to put a price on art
Hundreds of thousands of people have been targeted with offers to invest in art, wine, jewellery and shares.
In many cases the goods turn out to be worth only a fraction of what investors pay, as conmen play on ignorance.
The government said it had closed down at least 17 firms operating scams in the past three years but warned consumers they must remain vigilant.
As a rule of thumb, unsolicited approaches from crooked investment firms tend to be accompanied by exaggerated claims about the likely returns.
Often the goods turn out to have little or no investment potential, or may be sold at such a high price that a loss is almost guaranteed.
"The operators of these scams are very persuasive and it is often people who can least afford to lose the money who are being targeted," said Consumer Minister Gerry Sutcliffe.
Scam avoidance tips
Beware unsolicited investment offers
Always check out investment firms
Beware claims of 'guaranteed' returns
Never give out bank details until you are sure the firm is bona fide
"I want anyone who is tempted to invest their money to think very carefully about anything which sounds too good to be true."
In one recent case, an art company offered customers the chance to invest in art prints that would be leased out to companies to decorate their offices.
Customers were told that prints had the potential to increase significantly in value and could be leased out for 10.6% annual return.
However, the Department for Trade and Industry (DTI) successfully applied to have the company wound-up after it discovered that the art was being sold at hugely inflated prices and was not being leased out to businesses.