In our MLS system in Sacramento, we are not allowed to market a property before it is published in MLS, unless we obtain an MLS waiver. But that doesn't stop the sellers from chatting with neighbors or the real estate brokerage from circulating listings internally prior to publication, among other things.
Sometimes, I've even had sellers say they wanted to exclude a certain party from the listing, a party who has expressed interest when they decided to sell. Usually I prefer they settle up with that person before I extend a great deal of effort on their behalf. If I don't think they will act, I may also exclude them from the listing for a week or so. You can read more in my personal blog today at this link: Following Up On Buyer Leads Before a Listing Hits MLS.