What is considered 'scamming' is going to be very subjective from person to person. I think your are correct in what people mean when they say it ('sending a purposefully unfair trade'), but personally I do think people tend to over-use it and end up calling essentially any unfair trade a scam. Just be aware when you see people calling something a scam that they may just mean its unfair, I've seen plenty of trades sent from someone who just didn't know pet values get called a scam cause it was unfair. The official site stance is that a trade is only considered a scam if the other user is lying to you about a pet's value or otherwise provably misleading you. Those sorts of trades are reportable.
As for avoiding them, I would suggest utilizing the resources over on the
Trading Guide board, there are threads with lists of values for high value pets, threads for asking other people's opinions on trades you received, asking about pet values, etc. If you get a trade that seems suspiciously 'too good to be true', such as someone offering you a bunch of pets for a single one of yours, just double-check over on those threads that your pet doesn't have any special/high value that you might not be aware of.