1. Absolutely, someone should have offered you a seat.
2. I was horrified, a few years ago, in a Yerushalayim shul for Megillah where there were so few seats in the ezras nashim that there were more women standing than sitting. Still, those girls who were early enough to chop a seat did not offer to yield to the older women who were standing.
3. Once upon a time, I was shopping with a coworker lunchtime, and the clerk gave me the wrong change. I pointed out the error and she took care of it. My coworker asked, “Aren’t you angry? She tried to cheat you!” I have just as often had a clerk hand me too much change as too little. Bottom line is, I need to count the change every time. But for my own peace of mind, I’d rather assume there was no harm meant.
Brooding over what happened in the past only hurts you. Learn from it – be more sensitive to other’s needs, teach your own children better middos, and if you really need a seat, ask. Sometimes people are just too self-absorbed (or too intent on the davening?) to notice.