The buyers agent are working in the best interests of the buyers. It is normal that they would want to know the answer to that question. As long as there is no material defects to the property, there is no explanations due. It is up to the Buyers Agent and the buyer to decide the best terms and offer they want to place on the home.
But here in Illinois, I am not required to disclose why the buyer withdrew their offer. Even if they withdrew because the roof is 15 years old. As long as the roof is not currently leaking, I am not required to disclose that the buyers decided they didn't want a 15 year old roof, and the sellers weren't willing to credit them, for a brand-new one.
If the roof is leaking I am, of course, required to disclose that. But as long as the system is in working order and performing the function for which it was designed, regardless of it's age or condition, it is not an inspection issue in Illinois. This would apply to roofs (still holding out the water, even though it's 35 years old), a working 30 year old furnace, a functioning 12 year old water heater... well, you get the idea.
That doesn't mean we can lie about their age... but it does mean that we are not obligated to replace them or credit for them as a condition of the inspection. We may "choose" to, in order to hold the deal together... but we are not required to.
As mentioned below, you and your agent are obliged to disclose any defects uncovered by the previous buyer's inspection. The details of your negotiation do not have to be disclosed; if your first buyers wanted $25,000 to fix the roof and gutters, and you thought that unreasonable, you need only disclose the condition of the roof and gutters.
However. There are often times when it would be useful to explain why the last buyers backed out. Discuss this with your agent; it's my inclination to let your agent disclose anything that will be to your benefit.
All the best,
Recommend that you discuss this with your agent and her broker of record.
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Such as: The buyer got cold feet, or it just didn't work out. Sometimes if the inspection was very difficult, you can say "the buyer was expecting he was buying a NEW home". It is a bit of card-game, and you do need to keep your game-face on.