This agent is doing what all agent training is telling her to do - Call FSBO's and Expired listings "They are the low hangning fruit" that you have already "raised your hand" saying you want to sell. Tehre is a six year study out there by the NAR showing that something to the effect of 94% of FSBO's eventually end up lisitng with an agent, and listing with an agent can NET you about 11% more than selling FSBO. (I can't find the exact study right now but it is out there, and the numbes should be close, i'm going off the top of my head.) So now this agent is following up with you, hoping you get frustrated with no showing, unqualified tire kickers, etc. and end up listing with them, like the other 94% out there will.
Agents want listings period, they are leverage, you don't have to sit in a listing for it to sell, and yes people call off the sign and if they don't have an agent then bam, you(agent) have a new buyer. The thing sellers don't know, or realize is that Open Houses are not for the seller, they are for the agent. Agents do them to get unrepresented buyers walking though them, so if they9the buyer off the street) doesn't like this current house, they are more than willing to show them other houses in the area just like yours. Do open houses sell houses? sure I'm sure they do, sometimes, but they are a "dog and pony" show for sellers - it's another lead generation activity for the agent hosting.
Flat fee MLS. Are there agents who avoid them? Yes, for all the other reasons stated above, and also because you have "shunned" listing with an agent. Now will anyone come out and say that? No, do they do it? well... one would never be able to prove it. It's just easier to work with another agent, and with the ammount of inventory available, why would anyone want to work and harder than they needed to if they were truly still working in their client's best interest. I know that you have the paperwork and have done this before, and I hesitate to say this, but 80% or more of the agents out there only do about 2 transactions a year, and may not know how to handle you.
As far as craigs list goes, I'm not sure there is anything you can write in your ad to stop the realtor calls, just part of being a FSBO. All you can do is just be the best priced home that is also in the best condition and eventually it will sell. It's a price war and a beauty competition out there - you have to be the cheapest date to the prom. Period.
Just thank the realtors for calling and tell them that they can have your business with an approved offer.
Again, all the best!