This is where I come down on it. Expect the worst, be pleasantly surprised if it isn't. Before the season ends, I will hit close to 200 of them. It's an addiction although it is also entertainment. The bear went over the mountain, to see what he could see and all that. I've had some spectacular finds doing this, some were even audio.
This weekend I had a similar experience as the OP. The ad listed and pictured a lot of hand tools with a 9:00AM opening; I got there at 8:30AM and wasn't the first. Virtually everything I'd gone to see was absent. When I asked about it he told me he'd sold it during the week. Someone drove by and saw him staging stuff in his garage and came up & asked about it. Just the way it is, there is always another sale.
In the early 70s my mom had a yard sale. Back then it was put a sign on the light pole and run a newspaper ad. She actually got a phone call several days before about what she had for sale. Since she did not list her phone number it had to be the person that took her contact info at the newspaper that called or a friend of that person. We also had people knocking on our doors during the week wanting to see what we had. This behavior is nothing new.
Finally; last summer I hit one and made a driveway pile. This was at the direction of the seller, "Put everything there and we'll work out a price when you finish." I continued to peruse then turn around to see this little worm with a full armload of the stuff I sat aside. Paraphrased, I went up to him and told him I was going to buy it. His reply, "If you haven't paid it isn't yours.". At which point the seller said that was "my stuff" and he again comes back with "Why do you care who you sell it to?". It was truly surreal but I managed to regain my composure and made an offer of physical violence delivered to his face which he could not even come close to blocking; full as his arms were. He put everything down and retreated with his girlskank F-bombing me. The last time I'd been in a fight was when I was ~18. Yard sales bring out the best in everyone or not.
Ah, the joys of Saturday mornings.....