Startup and shutdown
If a world is renamed, it causes an "IllegalArgumentException: Invalid world name" error
Firstly, stop the server! Then delete the plugins/DuckShop/chests.properties file.
Start up the server again and re-link all the chests.
Chest links aren't saved
Make sure you stop the server using the stop command.
If you just click the X button to close it, nothing gets saved properly and files could become corrupted.
Trading
Cannot trade damaged tools, weapons or armor
This is normal, to stop players from selling diamond pickaxes with only one use left, for example. Check the examples page for examples of valid signs.