Open market value is the estimated amount that a property would sell for (exchange contracts) between a seller and buyer on the date of the valuation.
In the opinion of the valuer, it is the probable price which a property would be expected to achieve on the day in a open fair sale environment.
All auction properties are required to be appraised (valued) before going into auction so whether buying or selling, you'll have an independent valuation of the property to indicate open market value.
The open market value, based on the valuation, will also be used to suggest a guide price for the property at auction. The guide price, in-turn, can indicate a suggested reserve price.