There is the legal answer and then there is a practical answer. Pursuant to NRS 118A.330, the tenant is required to allow the Landlord to peaceably enter the unit to inspect, make necessary repairs or exhibit the unit to prospective or actual purchasers. However a landlord cannot abuse this right and (except in case of emergency), the landlord shall give the tenant at least 24 hours' notice of intent to enter and may enter only at reasonable times during normal business hours unless the tenant expressly consents to shorter notice. This is the legal rights of the Landlord.
Of course, you have indicated that you have developed a good relationship with the Landlord/Seller, who may be mortified by the bad form which his agent is self-servingly showing to you. You are not merely a tenant but a prospective (and the most obvious) purchaser for the property. You would be within your rights to appeal to the Landlord/Seller to have their agent show better manners.
Answered on Jan 22nd, 2013 at 2:33 PM