(MONEY Magazine) – YOU'RE SCANNING YOUR CREDIT-CARD BILL and you notice a charge for an item you can't remember having purchased. Sure, a couple of weeks ago, a telemarketer tried to sell you something over the phone that you agreed to try "free for 30 days." You would be billed at the end of that period, he said, but only if you kept the product--and you never gave him your credit-card number.

A few days later, the merchandise arrived. But you didn't decide whether you wanted to keep it because you had a month to make up your mind. Within a few days, though--and well before the end of the 30-day period--your credit-card bill arrives, complete with the charge for the item.

You can still return the merchandise, and if you do, the price will be credited to you. But even if you hurriedly haul the item to the post office, you could accumulate several months of interest charges before the credit comes through--interest that may not be refunded.

What you probably didn't know during your conversation with the telemarketer was that he already had your credit-card number, having purchased it--along with other information about you, such as your name and telephone number. And what you never suspected was that you would be charged as soon as you agreed to "try" the product.

What happened to you is a dubious practice becoming all too prevalent in the credit-card industry, according to Robert B. McKinley, president of RAM Research Group, a Frederick, Md.-based consulting firm that specializes in credit cards.

Buying and selling customer lists is a common business practice. But lists that include your account number are a growing phenomenon. While selling your credit-card number isn't illegal, it raises concern about the potential for abuse by unscrupulous telemarketers, who hope that once you are billed you won't bother to return the merchandise.

If you find yourself charged for an item you do not want, get to the post office fast--and use registered mail so that you have proof of the return. If you keep the item longer than your "trial" period, you'll make it difficult to win your claim that you were charged for an item you did not want.

To protect yourself against future unauthorized charges, get out your pen. Many credit-card applications now carry a line near the bottom that you can sign if you don't want your name and personal information sold to marketers. Signing will not affect your eligibility for credit; the only drawback is you may miss out on offers that genuinely interest you.

Current cardholders may telephone their issuer and request that their name not be included in any marketing lists. Still, some issuers may not honor your request. In that case, cancel the card and find an issuer who will.