"Preparing 1120S data"
Then you have nothing to enter for this sale that is between two private parties.
"and how would I handle note payments from ABC Corp on behalf of Person B?"
The corporation doesn't make these payments. The corporation would have Distributions to the shareholder, and the shareholder will do with the funds whatever that shareholder needs or wants to do with the funds. And that assumes this would be after Reasonable Payroll has been paid out for that shareholder, for services performed for the business, of course.
This new shareholder can have pledged the ownership of the corporation for security purposes, but that isn't the taking of a loan from the corporation and the corporation didn't just buy itself out of the prior ownership.
"Level Up" is a gaming function, not a real life function.