I sold a parcel of CDs to them and they paid up as promised (all low prices, as I already knew and they had quoted). However, I later sent them another package that I'd pre-registered with them, all items and prices agreed etc., but they refused to pay for two CD singles as they claimed they only accepted albums (then why did you quote based on the barcodes I entered?) and also docked the money for two 'missing CDs (erm, they're in the box, inside the packaging - I double-checked them five times).
They only short-paid me by about 20p in total for those four items, which actually seemed to make it even worse - so very petty indeed.
I sent a lot of DVDs to webuydvds.com. I don't know if they're still going, but if they are, avoid, avoid, avoid. They would offer you a few pence more for each DVD, but then take forever to pay you, after constant chasing. IIRC, they also reduced the price they paid out for some DVDs on account of their being in poor condition - still in their shrink-wrapping. OTOH, webuybooks.com were very pleasant to deal with - and they don't just buy books.
Bear in mind, though, that if you use their website and they offer you more than £1 for any item, that probably means it's very rare and that you might be able to get £20-£30 on eBay for it, so hold those items back. I think they work on multiples values, so if they offer you 5p for an item, they'll expect to sell it for £1, £1.50 or so - fair enough, it's not worth your hassle for that; but if they offer you £1, the x20, x30 (or whatever it is) multiple still applies, so sell it separately yourself!