A "fence" is old-fashioned slang for a person who buys and sells stolen goods. This might be the right register for "nepper", as it seems to be a term that was often used in connection with 1920's Russia. You can look up "fence (criminal)" on wikipedia, or look up "fence" in pretty much any English dictionary.
And by the way, "nepman" does exist, but seems to be mostly used to describe individuals from 1920's Russia.

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Note added at 3 hrs (2008-08-13 12:08:29 GMT)
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Of course, this definition of "nepper" deals with an "orgazda" - i.e. a "receiver of stolen goods". If you're looking for something more generally describing a car dealer who is not to be trusted, then you may want to go with one of juvera's suggestions.

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Note added at 9 hrs (2008-08-13 18:34:04 GMT)
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The term "shark" comes to mind, but I'll see if I can think of anything else.

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Note added at 2 days1 hr (2008-08-15 10:04:34 GMT)
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Since you're looking for a slang/colloquial term for an unethical/dishonest car dealer, I would go with "car hustler". You'll get quite a few direct hits if you google "car hustler".

Here are some definitions:
'hustle' = "to sell something...by energetic and especially underhanded activity" (Merriam-Webster Online Dictionary)
'hustle' = (slang) "to earn one's living by illicit or unethical means"(Dictionary.com)
'hustle' = (slang) "to induce by fraud, pressure, or deception, esp. of inexperienced or uninformed persons, to buy something" (Dictionary.com)
'hustler' = Slang. "a person who employs fraudulent or unscrupulous methods to obtain money" ;(Dictionary.com)