Hi Laura_Florida,
I don’t think there is a formula that can calculate this because it will really depend on the person and what they are willing to pay, on what else they have available and are comparing it to, their confidence that it will be a good match for them in terms of PPP (which they can test if they are local), their trust in you and it’s condition (which they can also assess to some degree if they are local based on post #2 here although your mattress doesn’t have a zip cover that they can look inside it), and on the risk of having no options available (outside of selling it again) if it turns out that the mattress isn’t a good match for them or if there is a defect in the materials.
Each person will have a different idea of “fair value” depending on their own personal value equation
Of course the number of responses would also depend on how many people that are within reasonable driving distance see your post.
In other words … it’s really a matter of how broad your audience is and on the “value” of the mattress based on the specific criteria of a potential buyer. I would also consider listing the mattress on Craigs List (for people you don’t know) or on any social media sites you belong to (for people you do know) as well. On this site most of the members here are more informed and will be comparing what you are selling to some very good value new mattresses which of course would reduce the price they are willing to pay for a used one.
Phoenix