I have a Longbow .177 walnut with shroud and a home tune, something of a sidewinder replica. I wouldn't take less than £300. Your sidewinder would probably fetch £50 - £100 more. But why would anyone want to sell the greatest and prettiest BB ever made?
Seriously, if I was starving and facing eviction I still wouldn't sell mine, that would simply delay the inevitable for a few weeks and I'd be Longbowless.