Grammar

"If your selling a car on Craigs list make sure you spell at least the name of the car correctly or you make yourself look stupid. Chevy isn't spelled Chevie..."

I suppose that people who cannot properly distinguish between "your" and "you're" don't make themselves look stupid at all. (Not to mention the fact that Craigslist, last time I checked, wasn't two words.)
Which brings me to a question/pet peeve. Why can't the average native English speaking, college-educated, monolingual adult in this country use proper grammar/spelling at a fifth grade level? I don't mean making occasional typos or mistaking "your" for "you're" once or twice because you're typing emails at 4 a.m., but really not knowing the difference to the point that you post 5 different for sale ads,(not that I'm counting), all with the same damn mistakes. Which means that you really don't know that cannot is one word, or that it's is not possessive, etc. I swear we learned this back in fifth grade, or maybe even third grade. Why aren't they teaching this and drilling this ad nauseam in the schools?
It's not like I expect people to know that peruse actually means "to read carefully" or that data is a plural noun. But you're/your? They're/their/there? Too/to/two? C'mon people. Every native English speaker over the age of 11 should have mastery of such basic grammatical concepts.