A buyer may inherit the CRM, quoting tool, scheduler, or billing system without inheriting everything needed to keep it running after the seller leaves.
Before closing, with the seller's cooperation, pick one important transaction and have the team that will remain take it from order to invoice. Track every vendor account, admin permission, export, integration, and exception that still requires the seller.
Give each dependency a new owner, then set a date to test whether the transfer worked.
For those who have bought operating businesses: which software dependency was hardest to uncover or hand over?