These pardons will be challenged and the Supreme Court will find them unconstitutional.

[tinrobot]

91. Because pardoning in exchange for something of value is itself a crime.

It is a federal crime if someone “directly or indirectly, corruptly gives, offers or promises anything of value to any public official” to “influence any official act.”

Pardons are most certainly "official acts". Trump seems to have gained something of value from that act.

So, if it can be proved that people withheld testimony or offered other things of value in exchange for the pardon, that itself would be a crime. You could prosecute that crime of bribery separately.

But this brings up a grey area. If exchanging valuable things for a pardon is a crime, what about the pardon itself? Would that be null and void? For example, would a murderer still be on the hook for murder if the pardon was obtained illegally? Or would the pardon stand and murderer only gets charged with bribery?

I think a pardon obtained illegally would also be illegal. But it is a very interesting question, and I think there's enough of an argument to get the Supreme Court involved.