Selling house - flat fee MLS or full service agent

[Dave Mustaine]

I'll be listing my house in Aurora, IL In a few weeks. I found a realtor with tons of current listings and past sales who will put me on the MLS for $100. Very high volume realtor. He also gets $200 of the buyer agent commission upon closing. He sent me the MLS input forms and I know all the answers, two other exact models recently sold. The only thing he does is takes offers and counter offers. I'm suprised he does this for only $300 total compensation. What's the catch? I'm planning to try this to save a few grand but want to make sure there isn't a big reason to not do it. Here is some info and then a few questions.

I'm not working so I don't mind taking all the calls to schedule showings and questions.  I bought a lock box at Home Depot.  I'm going to hire a photographer.   I have a lawyer. I already wrote my ad.  There will be no major home inspection issues.   House is 15 years old in great shape.  I know how to stage it.  I'm a good negotiator.  Is there anything else an agent could do for me that would be worth over 3k?    Is there anything I need to worry about? 

The flat fee listing agent said he will put my name, email, and phone number in the area on the MLS for the showings contact. If someone contacts me for a showing or questions, can I be certain they are an agent and not a thief? Im worried about giving the lockbox code to anyone who is not an agent.

Hmm, the listing agent said. If anyone contacted him outside of the MLS he would forward the email to me. However, I suppose a buyer or thief with no agent could call him and ask for my info. is there a fast simple way to screen people to make sure they are an agent? What should I ask? I don't have a fax machine.

Should I eliminate buyers with no agent since I have no agent to do the work.  Anything else to worry about?  Thanks!