Okay, I've been doing some researching on this, it appears I was mostly right. At least selling things on Sunday and holy days of obligation during special events is acceptable, so I assume buying things is as well. In regards to buying food, purchasing things at restaurant is acceptable, buy groceries generally is not, I assume, buying something for a quick snack while traveling is okay.
This video, while not catechetical, and somewhat off topic, helps explain things a bit. For anyone who is interested, skip ahead to ahead 8:25 for the specific of the 3rd Commandment:
I have also found some interesting articles on keeping the Lord's Day Holy. if I have time tomorrow, I'm going to take a look at these articles, and see if there is anything I can put into practice in my life right now, to keep the Lord's Day Holy.