While you are free to place an ad for your second floor vacancy, you must ensure that you follow the fair housing laws when creating your ad. It is a violation of fair housing laws to express a preference for a certain kind of tenant, or to specifically exclude protected classes of persons. For example, stating that only single or married people can apply would exclude families with kids, and that would be a fair housing violation. It’s a good practice when writing a rental ad to focus on the property itself and the amenities, not the kind of tenant you are seeking.