Present your credentials. What, if anything, have you done? Are you affiliated with any company or group? Most people will be wary of submitting to an unproduced, unconnected nobody.
Offer them some kind of incentive for submitting to you. Do you have the connections/resources to actually get their story on film? How will they be compensated?
Right now, it seems as though the writer has nothing to gain. They're offering you a script.
What do you have to offer a writer?