The broken heart and unbroken heart I always assumed meant the customer wanted to buy something for themselves or a loved one - the idea that the plant they had their heart set on is / isn't available sounds better though.

I've seen customers with a red $ and a frowny face (a lot of those when I had some high prices on my plants and it took a while to sell them). I've seen customers with a green $ and a happy face. Sometimes these appear before the customer even walks into the nursery. I don't know if it means they don't have a lot of money (or they do have plenty of money), or that the price I have on my plants is too much, or a bargain.

Before I bought the fountain I saw a lot of people with frowny faces and a picture of a fountain in their speech bubble. Now they all have the fountain and smiley faces. I've also seen some with pictures of birds, and I figured they were after a bird house. The more things I added to improve the nursery, the more customers came, and the faster I'd sell my plants, generally. But I haven't tracked whether those with frowny faces stayed around very long or not.

It's a pity you can't see which customers bought which plants. I've had plants sell when there's been nobody anywhere close to them, so it's hard to tell.