What is your actual point? You can't just throw out "consumer law" and expect that everyone will go, oh yes, that's a good point, because you need to give context to it.
First of all, I have a pretty good understanding of most aspects of consumer law, maybe not the really complicated stuff, but as a consumer and business owner, I've had plenty of first-hand experience as well as covered it in a couple of cerificate courses.
Secondly, the seller did not offer you the voucher, they did not facilitate or process it, and in fact - most importantly - they don't even know when a buyer has used one because they receive the full purchase price from eBay regardless, therefore they had no responsibility for it. Your dispute was unwarranted. Your dispute was like blaming the waitress because the chef put pepper in your meal when you requested no pepper (and let's just make it clear the waitress took your order down correctly), and throwing water in their face for good measure.
Thirdly, your dishonour fee is what is known as consequential loss - the vast majority of consumer protections do not include consequential loss.
