I have been sent what looks like a used filter for the hot tub from an eBay seller. It is in good condition and will probably act as a new one. The retailer claims it is a more expensive one than I ordered (and actually is, but the picture was misleading so I was actually expecting this one - the confusion came because I used a mobile app and didn't read the small print). He also says that it is purely shop-soiled and has never been used, even though there is a dead fly in it. The question is - as he isn't willing to refund my money AND pay the return postage - do I put this into dispute resolution with eBay?