I see a great deal of these spelling errors - "no" vs "know, "ur" vs. "your" or "you're", etc. in CL ads. I think it has a lot to do with the age of these people and the amount of texting that they do daily. People in their late teens, early twenties, and even thirties are so used to texting as their primary means of communication that they use "text speak" in all situations (that explains the lack of capitalization and punctuation as well). I don't even think they stop to think about how uneducated it makes them sound.
Of course, there are some that are just illiterate.
Regardless, it does make for some entertaining reading! I really do wonder if these people actually get calls.