As Steve says what you are trying to claim for is consequential loss. However, for a court to look at this one of the first questions that would be asked is "did you give the seller the opportunity to rectify the situation?"
From what you have written, this does not appear to be the case and as a result, due to either not checking the item when it arrived or there not being sufficient time, or both.
Under the terms of eBay transactions the seller is offering to refund the cost of the item and the postage paid originally. When you bought the item, it expressly states in eBay rules that returns are at the buyers expense and the seller does not have to refund these costs. Some sellers do as a good will gesture, but they are not obliged to.
Given the volume of transactions that this seller makes over 2659 last month, there is going to be the odd issue with the wrong item being dispatched. The 14 negatives equates to 0.527%. You obviously looked at his % score before ordering and decided out of the 95,000 items sold that 99.6% was pretty good and bought.
Given that the most you could reasonably claim for is the difference between the two costs (11.76 - 8.41 = £3.35) incurred plus your postage say (£1.95) to return the item, (£5.30), you are better chalking it up to experience.
I suspect though that even if you had looked at the negative feedback you would still have bought from them, as there doesn't seem too much to put a buyer off.