I've been doing a mental check of my home and I would be very hard put to raise £400 per month over a period by selling my non-essentials.
I'd get several thousand for my cars, which if I were claiming JSA, I would not declare to them, unless it was income-based JSA and not the 6 month contribution based.
If you are on any means tested benefit you should declare all monies you receive, but really would that include money raised from selling you car, or flogging your XBox and games to Cash Converters?
I don't know the answer, but have hopefully raised a question for those that do know.
As far as I am concerned selling your unwanted items on eBay is no different to flogging them to a second hand shop.