If it was listed as Used on eBay then you’ll likely be able to get your money back because it’s not fully working. If it was listed as Parts or you bought it from a less regulated platform, on your head be it.
As for repair, it depends what’s wrong with it. It sounds like some of the switches might be dead, but if they are then you’ll want to source replacements, but from where? Do you have any soldering equipment/skills?
It might be a fun repair project if you’re into electronics, but if you weren’t… then better luck next time.
I imagine it’s the fear of missing out on something even better. Think about it like this: If switching to this new board was so good then it’s entirely possible that there could be one that’s even better, so you might as well try as much as you can since there are effectively unlimited combinations of parts.
I assume it’s because you also added an image to the post? I agree it’s not exactly intuitive, but people tend to post either an image or a URL to a page, not a combination of any of those.