Bukkit

If you get errors, search if they are in the list here. If not, check commands and tickets. If you still have not found a solution, create a ticket and do NOT write a comment! You maybe will not receive support on comments, they are for commenting!
Also have a look at the server log!

"Not enough money"

• Check if you have Vault installed properly
• Check if your economy works correct
• Check the server log if Vault hooked into your economy
• Be aware that Vault is unable to handle multiple economies!

"No permissions"

• Check if your permissions plugin works correct
• Check your permission nodes, they are case-sensitive!
• Check if you have Vault installed properly
• Check the server log if Vault hooked into your permissions plugin

Errors on sign interaction

• Check out if an error log was created
• Signs-file could be corrupted, try it out with a new sign

Block placed on sign interaction

That is not a bug, it's normal block placing.

You can buy/sell the same items multiple times

If you receive no error log this is a bug in Bukkit, because the inventory isn't updated properly every time.