Mate, if it's in working condition and okay cosmetically then an Amiga 3000 is worth more than £30.
I've been looking for one of these myself actually.
Edit: Also, I think eBay is your best bet for these big-box machines (especially in the UK). Amigas can really push for high prices on eBay auctions these days. Just start it of at 50/60/70 quid, show pictures of everything clean/working and that the battery is fine.
Oh and post a link to the auction here. I think you'll get a good price for it. I might bid on it myself!