The main reason your not getting anyone interested is because of the scratches on the corners that look like it has been dropped.
N900's do not like being hit on corners as it rattles everything inside, if you read up on the problems with the device you will soon realise why poeple are not interested, sorry to say but giving you clues here ok.
I'll be in the UK end of June, and if anybody in the UK has a decent s/h n900 they don't want, I'd be interested. I want one to mess about with (as I use mine as my main phone, I try and keep mine as stable as poss.). I'm not interested in paying silly money for it, as I bought mine a year ago for about £250 and figure that s/h now they are worth about half that.
I took mine on contract for 2 years, back when it was hotest device on market, with extra "device fee" boy was I wrong I looked up prices, they are rock bottom compared to 500GBP i will pay until 2012... Good luck selling!