The investor buyer has an accepted contract on a property, and plans on listing it either for sale or for lease. Let's assume that the property is vacant. Must they wait until they close on it and have title, or can they list it now?

Certainly, unless the contract they wrote with the seller forbids that activity the buyer has the right to market the property. Yes, it would be better to obtain the sellers permission, especially if the buyer wants access to show the property, but again, that is not absolutely required.

A buyer in escrow, anticipating a successful close, has an equitable interest in the property. While they do not have title to the property, an equitable interest provides them with certain rights, which includes the right to list it for sale or lease.

Any such listing, and certainly any contract they execute with a buyer or tenant must contain a disclosure that the seller is acquiring title but does not yet have title, and a contract for sale or lease should be contingent on the seller or landlord obtaining title. There should be no prepossession, a tenancy could not begin until the client owns the property, and while a buyer might be allowed to do inspections and a walkthrough, they would not be allowed to begin any work or move anything into the property. If a lease requires the landlord to redecorate, repair, paint or clean, that should not commence until after they own the property.

Be sure to discuss any potential transaction such as this with your broker.