If the lease does not clearly lay out the notice period for showings, a tenant could make unreasonable demands such as requiring 3 or even 5 days’ notice for any viewings. This would dissuade the majority of buyers from viewing the unit in the first place. In the case of overseas investors who travel to NYC at short notice to view apartments, an unreasonable notice period would make it practically impossible to accommodate these types of showing requests.
Furthermore, most investors will not make any extra effort to accommodate a showing for a listing with a tenant who is making scheduling difficult. A foreign investor usually flies in, views whatever properties are available to schedule on short notice, and then decides to submit an offer on one or more of the units they viewed. There is rarely any emotional attachment for investors to any particular listing that would make them patient enough to wait around for a tenant to confirm an appointment.
If a tenant is really uncooperative or resentful, they could essentially sabotage the entire sale process by routinely declining requests for the listing agent to hold general open houses on Sunday. Or the tenant could demand that any open house be ‘by appointment only.’ Any listing agent in NYC will tell you that not being able to hold general open houses is a significant obstacle to any sale, as it essentially boxes out walk-in buyers.