Spelling and grammar mistakes do annoy me and, although I would never post to correct someone, I do tend to pass judgement on that person in the privacy of my head. I would certainly never buy something from the classifieds through a poorly written advert. As this is a forum, rather than IM, there isn’t the time pressure to respond, so it isn’t hard to spend an extra 30 seconds checking over what you are going to post, particularly as most browsers now have spell checkers.
Maybe it is snobbish, but as far as I’m concerned if you think something is worth posting about, and you want others to take you seriously, then you should write like you think it matters.
Edit: The exception is rants – these must always be poorly spelled with bad grammar and random capitals. It’s just how things are done.