Looking at the seller's other items, many of which can be clearly attributed, because it says what they are on the back, made me think about what we did before Ebay and the internet. One would go into a little junk shop or to a market stall in Portobello, or somewhere similar, (no offence intended towards the seller) and rummage around for this sort of stuff and on a vase such as this there might be a sticker, with a description such as Powell, or Powell? or there might be nothing at all. In either case one would enter a dialogue with the seller and ask what it was and how they knew it was that and one would get a feel of whether they knew what they were talking about, from their reply and quite possibly their body language as well, and then one could take a view as to whether it was worth buying! Most dealers would agree on purchase that if you found out it wasn't as described, they would take it back within a reasonable period.
Of course there was always the chance that the stall-owner didn't know what it was and that one could use one's own knowledge to buy something rare and unattributed for less than the market value!
Now that everything gets written down on Ebay, I often think that we assume that every seller should have superb writing and descriptive skills and also a degree of expert knowledge that probably isn't there. I am not really sure whether this is a reasonable expectation, but I would agree with a viewpoint that the sellers who can describe their goods clearly and accurately are more likely to achieve sales.
I do agree with Pip that the first thing to do is to ask the seller to take it back, since it was not as described and I also think that if you are on a tight collecting budget then Â£21 can be an expensive mistake. It is all relative.