I think it just depends. I'm not like a total stickler for spelling but I also can't read posts that are all completely spelled wrong, in one LONG paragraph, with commas placed everywhere, no periods, and total run on sentences. I wouldn't say anything but half the time, I just don't read those posts and skip over them. It doesn't have to be perfect, I don't even care when people don't capitalize things and write in all lowercase, as long as it's easy to read and understand, no run-on's, etc.
As far as craigslist and stuff, yup, makes you look sooo uncredible if you write terrible. I've seen so many "chiwawa's" for sale.
If you are gonna adopt out or sell your BYB pups, at least spell properly.