I’m wondering how eBay is allowing the sale of gas fires
Good question! As far as I can tell it isn't illegal to install a used gas appliance, it's just a question whether any registered gas installer would be willing to do it. I can understand their reluctance. It might help if you are able to find and download full installation instructions for the exact model you have bought. All you can do then is to ask around.
eBay did warn you in the user agreement that "they do not guarantee the existence, quality, safety or legality of items advertised, or the truth or accuracy of descriptions". It's important to do your homework before buying things here.