'Receiver'

[sassamo]

I'm not sure which mod this is, but I've seen one that would make a chest pick up items that drop near it.  Maybe would be abused for farming, but would make a great way for users to donate items.  Maybe only mods could set the property for a user.

On a similar note, but not sure if there's anything like this, a block that could generate a signal when 'paid' a certain amount.  This could be used to automate user markets, but also to automate 'mini-games' so the owner doesn't need to be online to run it.

[joshskiizz]

This is actually a flag on LWC, I believe. I was told it use to be allowed until, I suppose, abuse of the tool became common and thus had to be disabled. I don't think this will be returning anytime soon. I personally would enjoy this, but alas, it's disabled.

[Nyssa]

There was a dupe bug with it which is why it was disabled.

[sassamo]

for the other half of the suggestion, something like this:
http://dev.bukkit.org/server-mods/redstone-pay/
but maybe someone knows of a more developed mod, or maybe even if the current mod used for money can do something like this.