The tenant is right to deny access under the circumstances described. The landlords wants to accept rental payments while at the same time deny the tenant full use of the property to accommodate the landlordís needs to sell the property long distance. As others have stated the real estate brokerage should make appointments with the tenant (lockbox unnecessary), and the tenant should be allowed to be present during the showings to protect their own interests.
In addition, most property owners should require that the prospective buyer be mortgage pre-approved for purchase in order to view an occupied premise. This limits the number of unqualified (or only mortgage pre-qualified) interested purchasers viewing the premises without the ability to confirm purchase and minimizes the unscrupulous criminals posing as potential purchasers.