A local fellow in my area is will to sell BitCoins for below market price in exchange for cash. I can't help be wonder if it's a scam. Is there a reason why someone might be willing to do this?
Some possible reasons
He's an idiot/uninformed
He's willing to sell low because he wants to promote adoption
He's got a very low Net Asset Value (very early adopter)
He wants to unload a lot of coins quickly
He is trying to avoid records/taxes on this transfer
He's not willing to wait for the traditional fiat system (time preference)
He only has a high fee method of withdrawing fiat, so it is not actually a loss for him
He's got stolen/tainted coins
He's going to steal your cash
There is no official system in place to assign taint, but if they are from a highly publicized theft and not mixed before they are sold to you there may eventually be people in suits knocking on your door wanting to sieze them or asking awkward questions.
In 7 of these 9 reasons there is no real problem for the buyer, but if you want to avoid stolen coins or getting mugged you should ask some questions, get a copy of his ID, and do the deal in a public place with witnesses.