I'm selling my 2 1/2 year old 20 cm Tempur California King,just replaced with a Sealy. I'm try to sell it. Paid for it back in april 2005 € 1.750 ( about 2200 dollars back then). My mom suggest me to sell it for no less than € 900,but i know that a mattress is a very personal thing,I should come up with a good price to make it more appealing...what do you think ??? Any suggestion ? Do you think € 500 is too low??

Just stating the obvious here, but how much you paid for something is irrelevant. It's worth what someone is willing to pay for it, and that's not going to be much for a used mattress. Check eBay past sales, and Craigslist or whatever online "want ads" you have in the country in which you live, to see what others are asking.

Your mattress is what Tempur-Pedic now calls the Classic, or one of the two models below that?