Chrissa,
How old is the sofa bed? If it is before 1988 it won't comply with the fire safety regulations, so the auctioneer wouldn't be able to sell it.
It is a fact of auctions that soft furnishings do not make much money, I bought my parents two large sofas, an armchair and foot rest at a local auction for £100, they had been supplied new a couple of years earlier from a local shop that is very expensive, the original cost was £7000!
I would agree with the auctioneers valuation of £30, and say that is probably at the higher end of his estimate.
If you look at the fact that it has already been transported to the saleroom and back (even if it was in your own car) it has still cost you time and money, the auction house when selling it for you would charge you typically 10-20% commission, so making your £30 less, plus the fact you would be transporting it to the saleroom yet again (eg more time and cost)
Whilst you are in a bad financial situation and as is your sis, i suggest, you sell it to her for the £30, as this is better than the auction house estimate due to the fact there will be no commission to pay.
One further point, the person who suggested that the auctioneer might be trying to stitch you up like arther daley, clearly doesn't know how auctions work, it is in the auctioneers best interest to get the highest price possible, as he then gets more commission.