Hey there I would agree with the first poster, try selling it for 450-475 dollars. That would seem more reasonable since you did use it for a year, I would also recommend polishing up the case and cleaning out the dust prior to posting pictures, a cleaner computer would probably draw more people compared to a dusty computer with fingerprints all over it. I'm assuming that case does have a glossy exterior, which is why I recommend polishing it, if not then just clean it up a bit. Brett does speak the truth, I myself have tried selling video cards on my local classifieds website (kijiji) and boy do people lowball to the max even if I do post a good price such as $40 for a HD 5670 which I bought for around $90 dollars at the time. Despite the $40 dollar price people come to me asking for it for a mere $20
Anyways I would try posting it on ebay, perhaps try a bid with the starting price at the least you will take it. I am confident someone will bid and then it will turn to a bid war until the very last second. Too bad I'm too young to sell on ebay.
Once again you are true that people can go buy a better computer for 400 euros considering if its a custom build/ or even pre-built with newer technology and you do have a good point with the warranty factor. You could however have a chance at 400-500 for that build on ebay, I have seen some crazy bids on items on ebay. The OP's build does have a good CPU (2600) which would help it sell but the video card is a bit weak (GT 440) nonetheless there is always someone out there who is willing to buy it. That may take days, weeks or who knows how long and that is why you are right about it being hard work. We should also consider the OP has bought the PC for $1000 euros which is quite a lot and if he does mention that in the description it may attract people knowing they get 50% - 60% off.