What's stopping trump from selling more pardons after he leaves office,

[Bernardo de La Paz]

12. Person w pardon, even public pardon, can be charged in court. See Burdick SCotUS 1915

Charge any and all of the criminals and force them to use (and reveal) their pardon!

According to the 1915 Supreme Court decision, you can be forced to use your pardon in court. And using the pardon is to admit guilt.
https://en.wikipedia.org/wiki/Burdick_v._United_States

The pardon does not actually apply until the defendant proffers it in court in answer to a charge. Nobody can proffer it for them. If the defendant does not proffer the pardon, then they can be convicted.

So,

1) Prosecutors should formally ask every pardoned person if they admit guilt.

2) If they do not, they should be charged.

3) Then they would be forced to admit their guilt, formally, in a court of law, where it will be recorded.

The Supreme Court ruled in Burdick that a pardon carries "an imputation of guilt, acceptance a confession of it".[2]

Burdick v. United States, 236 U.S. 79 (1915), was a case in which the Supreme Court of the United States held that:

* A pardoned person must introduce the pardon into court proceedings, otherwise the pardon must be disregarded by the court.

* To do that, the pardoned person must accept the pardon. If a pardon is rejected, it cannot be forced upon its subject.

A pardon is an act of grace, proceeding from the power entrusted with the execution of the laws, which exempts the individual on whom it is bestowed from the punishment the law inflicts for a crime he has committed. It is the private though official act of the executive magistrate, delivered to the individual for whose benefit it is intended.... A private deed, not communicated to him, whatever may be its character, whether a pardon or release, is totally unknown and cannot be acted on.[1]