Does a buyer have to know (s)he's a buyer?

I've been burned or surprised a couple of times when it wasn't crystal clear that I was buying something. I'll explain.

1. A friend and I were staying at a hotel. We walk past the breakfast bar. A very enthusiastic employee points to the plates and says, "Go right ahead!" So we do. It was about halfway through the meal before we realized that we had bought a breakfast buffet. She didn't offer this information until we asked. (Presumably she would have called after us had we left without paying, of course.) I raised a stink, and we ended up getting the meals for half price.
2. We had a snowstorm last month, and there was a bit of a commotion out in the cul-de-sac. Someone had hit my car which was parked out there (since I didn't want it to get stuck in my driveway). There were three people: two people plowing snow, and a relative of my neighbor. My wife went out there to see what was going on, and during the exchange, the person plowing the snow said that he was over to plow the cul-de-sac and my neighbor's driveway. At this point, he offers to plow our driveway. My wife didn't know who actually hit the car, and since the damage was minor, figured that he was partially responsible and was trying to make nice. Well, afterwards, he waits to be paid. My wife refused, and I went out to talk to him. Turns out the snowplow guy didn't hit the car at all, and he was there for hire. Basically he was looking for another job, and perhaps capitalizing on the confusion to "sneak the job in." I ended up paying him, but did tell him that it wasn't at all clear to my wife that he was to be paid.

So now I'm (perhaps) hypersensitive to this kind of thing. So much, that when I went to the aquarium this weekend with my daughter, and when they tried to take our picture right after I entered the aquarium, I quickly gave the time-out sign and asked them if I had to buy the picture if they took it!

This kind of sales procedure seems shady to me, but is it allowable? I'd say "buyer beware" but it's almost more "beware, you might be a buyer."