Undoubtedly some sellers inflate the weight to try and make more profit, but I think it's mostly bad estimates. People in general are lousy at estimating size, weight, numbers and time. Unless you have a scale at home, it's easy to be off by a factor of two or three if you're just picking something up and guessing the weight. Most won't bother to take the goods somewhere to get them weighed.
I never had a bathroom scale until recently when my doctor recommended one to keep track of some medical issues. I found a digital one on sale for $10, and it's remarkably accurate. I've shipped a number of things since I got it and it's never been off by more than a pound. If you do a lot of shipping, it would be a cheap way to get accurate weight estimates, even if you don't care what your own weight is.
Like Rlwainwright, when I'm buying something I tend to make offers inclusive of shipping to avoid any problems or hard feelings.
As an aside, 25 years ago when the UPS limit was 50 pounds, I had a UPS driver that could pick up a box and tell whether it was 49 pounds or 51 pounds. Amazing.
David
