Have just read a guide that I would like to agree and comment on.  I bought a memory foam mattress on ebay and suffered for over 6 months with no sleep and headaches until I had to count my loses and throw it away.  It cost £200 and I contacted the vendor immediately after opening it to say it was so hard I could not lie on it - but his reply was if it is out of its wrapping he cannot take it back! Beware of buying these.