The owner of a certain type of item writes a shtar stating “I give ploini the exclusive right to sell item X from year 2013 and on,” and then signs it.
Is that sufficient for the shtar to be valid?
This will apparently not be sufficient.
A shtar can only be written for specific matters that a shtar is effective for. For instance, a shtar is effective for selling a field or land property, and for making a Kiddushin.
Likewise, a shtar is effective where it is the common custom to use contracts. For instance, a business contract will be binding if done in a manner that is recognized by the law.
However, in this case a simply signed document giving a person certain rights is not recognized by Torah law, and not by custom (I believe it would require the authorization of a notary to have any legal weight), and will therefore not be valid.
Please note that this ruling is not binding on the parties involved, and only if both parties accept an authorities arbitration is his ruling binding on them.