You can get a surprising amount of variance in weight, even on sets purchased direct from a distributor. I've seen minifigure cases differ by up to an ounce, straight out of the factory 'brown' cartons. Never had a complaint that a figure was missing from one. I do not deal in very old/valuable sets like the Taj, 10179, etc. Maybe these attract a higher grade of scumbag, The only opened and resealed sets I have ever received were from Toys R Us. All seals on one side of four different sets were removed, then reattached with Scotch tape. Not exactly a professional con job. They're all the correct weight, but I haven't opened them to check the contents. There's no point, since Toys R Us does not actually credit my credit card when I return items. I think they're basically honest, but hugely incompetent, which ultimately makes dealing with the company about as bad as buying from anonymous weirdos on Craigslist.