Where your "odds provider" isn't currently offering a price on a runner within a market or you do not wish to take further bets on that runner at the current time, rather than omitting that runner from your market, why don't you instead list the runner with a prohibitive price of 1.0001 (US -1,000,000) or a 'no offers' tag?

I believe that this would reduce confusion and complaints, particularly in markets where it may not be obvious to most of your customers which, if any, runners are missing.