What to ask for?

[MissCalamity]

I bought a duvet set off a shop on Ebay, checked before I ordered the material and it was 50% poly, 50% cotton.
Received on Monday and got it out of the packaging and put it in the washing machine, then tumble dryer.
When I got it out of the tumble dryer I thought it felt a bit strange & it's 100% polyester.
I really can't stand the feel of it (and god know why I didn't click when I got it out of the packaging)
Obviously as I've washed it I can't return it, seller has waffles about wrong stock errors but only offered 20% off.
It was only a cheap set (£14, as a back up) but I feel £2.80 back is a bit of a piss take when I will probably stick it in the jumble bag.
I know I can't return it but am so annoyed as the listing clearly states 50/50
Would it be worth perusing do you think?

Why can't you return it?
It's still 'item not as described'.
I would raise a case with eBay and ask them to decide (which they probably will in the buyers favour)

Put it back in the packaging and open a not as described case. Photo of the label included. Although in theory you have accepted it by washing it, you were not to know they had described it incorrectly so go ahead and open a case.

deffo not as described case; you have right to ask for full refund