Signature Sleep Essential 6-Inch Twin Mattress bought from Amazon for sale for $75 OBO. It was very lightly used for exactly 8 months. I'm selling it because I got a larger mattress and don't have space for this one in my apartment. It's very clean and the springs are still firm.

This is where I bought the mattress:
http://www.amazon.com/gp/product/B005A4OPS4/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1

You're more than welcome to come and inspect the mattress before buying. You must be able to pick it up from Mountain View.