Is it normal to be constantly observing people for signs they are annoyed, angry, in a bad mood etc?
I realised that I do this; I'll often pick up small, probably meaningless things and imagine it means something; I then get tense or anxious and try to be self-depreciating or avoidant.

I know it's part of social anxiety.
I find that whenever I perceive someone to be unhappy for some reason, I become avoidant of them. I also frequently imagine that they have a problem with me.
I recognise that this is probably more often than not not the case.

For instance. A good example is if someone is concentrating on something. Say they are doing something on a computer. They have that face, and I feel the need to tiptoe around them because that expression makes them look like they're irritated. But no, they are just concentrating.
Alternatively, I recently started working at a supermarket, and often a customer comes and they have an expression that makes them looked unfriendly. They won't say anything or look at you, and I assume they don't like me and avoid talking. However often, you'll ask them a question and start talking to them, and they'll be totally fine.

Anyway, I realise this is a totally unhelpful habit, but I'm not sure how to deal with it.
Is this something most people do, or is it a result of me being on the extreme end of the social anxiety spectrum?

>messaged seller on craigslist
>hasn't responded yet
>mfw
Am I overreacting? It hasn't even been 24 hours yet but I'm scared he just will never respond or it's already been sold