Seller won't accept return

Hello!
Did have a read through a few pages and couldn't find this here, many apologies if it's something that gets asked every day.
So, I was under the understanding that buying online/specifically ebay is covered under law and that sellers have to accept returns, for whatever reason, within either 7 or 14 days, which this was.
I've had a bit of a nightmare experience (first one for a while) from someone who is allegedly a private seller but seems to sell an awful lot of stuff - presumably staying just under the limits so they can not pay business rates or play by business rules. They refused to accept my return and because I'd clicked 'changed my mind' as the reason, they were allowed to close the case by ebay and not refund me. I'm happy to send it back and pay postage but it's not what I wanted. I did ask a couple of questions and I was pretty sure it was but unfortunately it's something that even in photographs looked like what I wanted, but it isn't. 'Ordered by mistake' seemed the best fit.
Had a bizarre call with ebay, some manager who sounded like he'd been watching too much Wolf on Wall Street and kept going on about how I need to 'outsmart the seller', I must 'never ever ever click on ordered by mistake but as item not described' because now 'I can't help you'. He said 'your last chance' is to open a case via paypal saying it's not as described. But that seems dishonest to me, I haven't checked carefully but it doesn't seem to be broken either, which are the only two options to open a dispute on there. It's the Internet, it's hard to know sometimes if something will fit, or what colour it is, or whatever, but I'd never worried about it before as I assumed the law covered me if I wanted to send something back. I thought that that law was there exactly to get people to feel safe buying online BECAUSE you can't see the thing or physically touch it or whatever.
I had no idea there was a difference between 'business' and 'private' sellers and that private sellers are told they don't have to accept returns if they don't want to. I thought by law everyone had to?
Mr Wolf just laughed and said 'I don't know, I'm not a lawyer, what can I say' but if they're ebay it's up to them to know the law that relates to their business and follow it properly, isn't it? Not laugh as if it's some quaint thing us Brits have that they find charming.
Also, since that's a massive difference, I wish they'd make it more obvious which sellers are private and which are businesses. I didn't even know there WERE two different tiers of sellers until today.
I'm confused, both as to whether I'm in the right here - I understand returns are a hassle, but I'd be losing out on the return postage with not much left over to show too (it's quite heavy), I didn't do it on purpose. I was reasonably sure it was the thing I wanted. Getting it, in the light, it's not.
I asked some friends and they all say that I should open the paypal case as 'not as described' and just get my money back, the seller has behaved in a kind of dishonest way anyway (trying to keep this short, but their behaviour and communication has been very poor). But I have a real thing about lying and it doesn't feel right to me.
But then neither does letting it go if those are my rights. Has anyone tried to get ebay to force sellers to accept returns when they don't want to? Or gotten them to refund you as a 'goodwill payment' when you mention the law or whatever? Mr Wolf said they don't give goodwill payments, but they definitely do, they gave one to a friend of mine after his own nightmare experience, when by some reason they didn't want to force the buyer to cough up, so they just paid him from their own money instead. I've been on ebay for about, 13 years (although I don't use it much), I thought that might count for something, but apparently not. Trying to tell some bloke who lives in the Philippines that you're really upset and ebay is about to lose a loyal customer because I no longer feel like I can trust online shopping has zero effect, he couldn't give a monkey's.
Also, if the law has been updated or changed, perhaps that's it, but as far as I know it's (they made me remove the link as I'm new, which is annoying, so you'll have to find it yourself - it's on the which site under consumer contract regulations, which replaced the distance selling regulations in 2014).
It was a buy it now, not an auction, and was under £42. I've seen £42 as a threshold in a few places but not sure if that means it's not covered or that's a different law.
Anyway, hopefully someone out there has advice. I'm thinking snail mail letter to their UK head office? I'd also be interested to know what other people think I should do - let it go or chase the refund. I honestly don't want it, so if I can't send it back it will go to a charity shop or the bin, which is sad, I'd rather return it to the seller so they can hopefully resell it to someone else who will want it. It's unused.