1. It is unreasonable to say that buy times or sell times cannot be reset, as the plugin itself has a built-in reset system.
2. Of course, you can only set their buy times and sell times for one player at a time, which is like you cannot use Minecraft's built-in /give command to give all players an item (i means all online + offline players). Assuming your server has hundreds of thousands of player data, after using this command, the plugin needs to read the data stored by each player and then modify it. I think your server will crash quickly without special optimization process.