Regarding the currency system, I'm not crazy about it TBH, but if we do it, I think it should definitely use emeralds, since that's what they're there for. Expanding their use beyond just villager trading into player trading seems only logical.
On the other hand, if the Allocator mod is SMP/Bukkit compatible, I can create trading stations that prevent cheating. 
Emeralds are not currency, they are materials, and would the the same as someone trading cobble for dirt. This doesn't stop them from using it as if it's currency, like gold bars or diamond, but it isn't.
It creates items that do not use stock assets, thus requiring client-side mod. Not happening.
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For those who do not understand how Essensials handles Economy, it does so like this.
Earning money:
- Trading with other players in person.*
- Selling items via Trade signs. Players will have access to Trade signs.**
- Selling items via Buy signs. Only server will have Buy signs, and even then only for a few things like gold and diamonds.
Spending money:
- Trading with another player in person.*
- Buying items via Trade signs. Players will have access to Trade signs.**
- Buying items via Sell signs. Only server will have Sell signs, and even then only for a few special things, If at all.
*:
- Players use commands to give their money to the other player, while the other player drops the items as usual.
- Both sides will get a text confirmation of the amount transferred.
**:
- When a Trade sign is created, it takes the items you have for sale out of your inv.
- To refill a sign, you right-click on it with what you have for sale.
- To withdraw from a sign, right-click on it with your hand.
- To get your stuff back from a sign, break it.
-
Syntax.
There is no way to cheat the system besides either scamming another player, which, really, rule 1... and if an admin sets up a unlimited cash loop on accident. But since all there would be is a buy for gold/diamond/emerald, and a sell for... stuff that isnt gold/diamond/emerald, that is unlikely.
