But these services could be worth it, especially for those with no credit or who are looking to rebuild credit, says John Ulzheimer, president of consumer education at SmartCredit.com.

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"You look no different than somebody with a big fat balance, " says John Ulzheimer, president of consumer education at SmartCredit.com, which sells credit-score monitoring services based on its own scoring formulas.

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"Margins on gasoline sales are already so razor-thin they can't offer you 50% off like a clothing store could, " says John Ulzheimer, president of consumer education for SmartCredit.com, a money-management and credit website.

WSJ: Gas Credit Cards Offer Few Rewards

What's more, the requirement to have a co-signer over 21 years old or show proof of income also can be easily manipulated by merely having a fellow student who is over 21 years old be a co-signer, says John Ulzheimer, president of consumer education at SmartCredit.com.

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