I got sucked into going to a Mary Kay party by one one my friends. I can't afford 12 dollars for mascara. I don't even wear make-up. I'll end up buying something out of guilt. That's how Mary Kay works. (I hope the avon lady isn't reading this...)
Mary Kay is a company that sells make-up. Or more precicely, they sell make-up in bulk to people, and they turn around and sell it to their friends. and their friends friends. and so on. It's just awful, really.
If you have to buy something, go for the foundation. It lasts longer and it's all you can probably reasonably afford. My mother tried to get into it. She thought that she would be able to run her own little business after she retired, but she went to this one home for one of her first parties and was devastated at trying to sell this poor young woman and her grandmother junk they couldn't afford or need. She found them a nice perfume and didn't push for anything else, because it's not in my mother to mistreat people like that when they clearly couldn't afford it. Was nice to give them a bit of something that made them feel happy and pretty, but I hope you pick something out that you would never really buy for yourself and keep to it. Don't feel forced to buy anything, I believe you get a few items for free for how much you sell at your party anyway.