Commercial brokers have their own MLS that they work through and will also list in on the standard MLS under Commercial Property, usually. Serious investors may approach them directly about properties that they have available and commercial brokerage companies also will have the ability to search their company websites. Your property can only be listed as commercial real estate if it is deemed so in its zoning.
Also, just as a side note, tax value has absolutely nothing to do with market value. This is not how appraisers or real estate agents or buyers assess the value of home and it is used solely by the county to determine what you owe them in property taxes, nothing more.