Hi Barbara - well there are two questions here really: one is, should you have bought the bowl. The other is, was it worth a lot more than £30.
The eBay auction you show is of a very pretty bowl, but I think the price is way out. A LONG way out. Imperial purple is very sought after, and very beautiful, but the pattern and shape does not warrant that kind of price. However, if the one you saw at the boot fair was in great condition, perfect in every way and had superb iridescence, then yes, it would have been a good buy for £30.
There have been some reproductions of the pattern - they are usually marked IG or LIG. You say the bowl you saw looked garish. I wouldn't use that to determine if it were old or not. Imperial's purple (made almost a century ago) can be breathtaking. Jam-packed full of colour and shimmering with blues, greens and purples.
The eBay auction is wrong in calling the item a rose bowl. It is not. It is simply a bowl. To fit the rose bowl shape, it should have cupped in sides at the top.
Hope I've answered your question(s). The bottom line should always be - if you like (love) it - then get it. If it turns you off, then leave it right there and walk away.
Glen