I have a one-person mattress for sale. I used it only for a little while (2 weeks, when staying with a friend and looking for my own flat) and the rest of the time it was just lying there waiting to be used, so I finally decided to sell it. It was Protected content , I bought it for Protected content a discount. I wouldn't say it's a good mattress to be used permanently,it's a bit too soft for my liking, maybe H2 or so. However, it is really good if you have some friends staying with you for a week or so. The mattress is over 20cm thick and is perfect for putting on the floor, is warm and has a cover which can be taken off and washed in the washing machine. I will sell it for 50 euros.
It's located not far from Konstablerwache or Eschenheimer tor right now. It's easy to transport even in public transport, two people will easily manage it. If you will want to pick it up on the weekend and will need help, I'll be ready to help you :)