"The problem with background checks being expanded to private sales ... is that there can be no regulation of private sales without registration, either retroactive or on a rolling forward basis."
I don't understand why. Buyer and seller meet at gun shop (FFL). The FFL holder runs a NICS check on the buyer. If the buyer is legal, he pays the private seller the agreed-upon price, the cost of the background check, and a little more for the FFL holder's time: $10 for the NICS and maybe another $15 to the FFL holder. That's $25 for peace of mind, and adds considerable inconvenience to criminals and just a little inconvenience for law-abiding citizens. Also, the law could and should be written so that the FFL holder and the feds cannot retain identifying information after the transaction is complete.