As a practical matter, if the seller knows that the buyer is not a prohibited person and trusts the buyer, I see no problem. But a private seller usually has no means of determining the status of a buyer, and so could be in trouble if the buyer misuses the gun. The laws generally say it is illegal to "knowingly" sell to a prohibited person, but if the state can't prove you "knowingly" sold to a criminal, you have no way of proving you didn't know. Even if the sale is legal, if things go badly wrong, "they" will find some way to get the seller.
(Think of the pressure that would be put on a private seller who sold the gun used at something like Sandy Hook! Every trick in the LE book would be used to put him in prison, no matter what the law says or doesn't say.)
As for tracing, any gun made or imported since 1968 can be quickly traced to the first buyer. And it is not very difficult to trace past that with the power of the federal government. Think you can pull that "I don't remember"? They will make you want to remember!
If you do sell, get the buyer's ID - drivers license name and number (check picture and description against the buyer), conduct the sale in a public place, have a companion, and be armed if possible. Make up a bill of sale; usually not required by law, but an indication that you made an effort to identify the buyer. There is some good advice here:
http://thearmsguide.com/2442/firearms-private-sales-how-to-protect-yourself/
Jim