Sadly yes scales can be messed with and some scrap yards are shady and down right thief's. In the Jnited States Scales are checked once every 3 months by State Police or similar in those states that dont have State Police. Here's the best thing you can do to protect yourself, buy a dependable scale my opinion is buy a scale for bass fishing or any hand fishing. They will weigh up to 50 pound and extremely accurate. I weigh 20 pounds at a time than add to a bucket or tub I've pre-weighed everytime. This will help you spot when something is off and sometimes it's not the fought of the scrap yard. These scales being used will fail on reporting the accurate weight without warning. But I also believe it should be law each day scrap yards need to test the scales every morning with weights just for that, like 50, 100, 250. This way scrap yards that are legitimate will find out right away or within 24 hours of something going wrong with the scale. This helps protects everyone, the buyer and sellers. I'm unsure of the federal laws for a scale tampered with but it is serious. If you cant see your weight at a scrap yard you sh will uldnt deal with them, 99% chance they're shady as hell. I would rather drive a hour or more for a legit yard