Retail stores do not scan serial numbers, they scan UPC bar codes for inventory purposes. Now, whether or not the manufacturers imprint the serial number within the UPC code is a whole other story. If you want to be extra careful, don't buy from a retail store. Go to a pawn shop; however, depwnding on what you want, the selwction may be slim picking at a pawn shop.
On another note, presume you are on camera anytime you leave your home. There is no longer any expectation of privacy when one is in a public setting. Any store you go into, whether retail or pawn, will have cameras. If you are really that paranoid about being seen on camera, buy from someone on Craigslist, but make sure you provide a fake name, phone number, and email address.