Another method of avoidance is giving fictitious information on a credit application so that it is hard to trace you. If the seller has valid driver's license and social security numbers, most people can be tracked down even if they have moved to another state. The price you may have to pay to make certain these numbers are right may be more than the value of the debt, so get a good idea of the costs before you decide how far to go in the verification process.
One quick test is to ask the person for a social security number. If they have to look it up in their wallet, unless they are very young or very old, the chances are they aren't who they claim to be. That's because most people use their social security number so often that they remember it.
Any professional thief would memorize this information and have it down pat, but there is a real difference between someone who lies to attain credit once in a while and a professional thief. Most people won't go as far as to forge a driver's license and get a fake social security number in order to get credit, and besides, if you are that sticky about it, they may as well steal from the store down the street. Very few legitimate credit-worthy people are sufficiently offended by being asked for this information to refuse to give it.