Using 'of' instead of 've' is a common trait amongst British people because of the way "should have" sounds in most British dialects. No one posts of when they should post have, only when the sound comes off as 've' abbreviated on the end of another word. For instance no one would ever say "Can I of a second opinion?" It's context Rayne. Your and You're is just plain lazy as opposed to a legitimate deep set habit. If someone says "I should of got that" you instantly know they mean "I should've got that" or "I should have got that" Your and You're changes the entire context of what's been said. Have and Of does not.