I'd like to use this plugin w/ vault to sell keys with X amounts of usages.
Example: Sell train tickets with varying degrees of usage
1 use , 5 uses, 30 etc.
time based usage, unlimited uses for x minecraft days

After the uses run out, the key gets destroyed

It can be cool but if the key gets destroyed then the padlock must also be destroyed.

No I don't think so. If it was for an "access" area the padlock should not need destroyed. The padlock would instead allow for different types of COPIES of the key to open that area.
So you would sell a copy of that key, and the seller keeps the original

Yes but imagine the following scenario :

• A player creates a padlock with its key.
• He uses it x times and then the key gets destroyed but not the padlock.
• The player is then unable to open the padlock and the operators have to go there and run /sk getkey.

Skyost I dont think you understand the point of it -- its for PUBLIC doorways and gates. The seller will always have a key to copy and distribute

The only people who are locked out are the people who need to buy one. It would work like a ticket or a pass