How much should i sell a used mattress for?

We have a mattress that we need to sell. Its about 6 months old and has some milk and urine stains. I am going to clean the mattress first before selling. I am selling everything the mattress, box spring, and the metal frame.
I dont remember how much we bought it for. I think it was about $200.
its a queen size.
The stains are from my son when he stayed with us but they are not too noticeable. I dont want to throw it away. Its in good condition except for the stains.