Well, this is the reason I came to this forum in the first place. I’ve noticed this too, and it’s pretty annoying.
If you try to sell copper for example, which at the time of writing has a stack of 231 thousand listed, you will sell it almost instantly. There is no way people bought 231 thousand copper in just a few seconds.
Another experience to back up my claim was when I were selling copper I had mined and someone added a few stacks 1c cheaper. When that stack had sold out, my copper didn’t sell instantly as it did moments before. This (I think) was caused by a bunch of other people adding more to the stack after I did, leaving me waiting for about a day for the copper to sell.
Edit: Of course, the downside of fixing this is that you won’t get instant money anymore. However, isn’t that why there are buying offers?