Buyers need to ask realtors to make sure a full set of keys is at their closing. The buyers should communicate with their realtor on the return of the keys, garage door openers, etc. at the end of leaseback. Always best to check with your insurance agent on how the leaseback might affect coverage for the buyers and new tenants in the event of a claim during leaseback time-period.
FYI, the tenants should also make sure they have coverage in the event of a loss as they are no longer owners of the property.