It's 15 days for the buyer to transfer the vehicle into his/her name, not 5.
Also, it's my understanding that they prorate the remaining time on the old tags for the new owner. In other words, if you bought a vehicle in June with tabs good through October, you would get a credit for 4 months on your new registration (you would pay the cost for 8 months, not the full 12). You still have to pay the cost for the new plates, though. I had this explained to me by two supervisors at the local licensing agency when they were trying to figure out how to do the "year of manufacture" plates on my Barracuda. I couldn't find anything about this on the DOL website, though, so it's totally possible they were misinformed (it did take them 45 minutes to figure out how to correctly charge me for using a '68 plate on my '68 Barracuda, after all).