The /sendmoney or /sm only tells the sender that they have given money to someone, it work well if the command told the receiver (if they are online) that they have been sent money, otherwise there is no way to track.
To pair with the above, a message that tells you when someone has purchased from your shop would also be useful (if you are online), that way you are aware of where funds are coming from.
As well as this a possible transactions log? For example /transactions would tell you your last 10 transactions, how much you have sent/received or purchased/sold.
Possible thoughts?