I'm afraid that this is just one f the risks of buying on eBay, especially phones.
eBay's own buyer protection only lasts for 30 days from delivery. We often hear from buyers whose phones have been reported stolen or are blacklisted after this time limit has expired.
Often buyers can get a refund through their credit or debit card issuers, if that's how they paid. You can still try, but I imagine that 18 months will be beyond any time limit.
You could talk with citizens advice - with a bit more information they may be able to advise you whether there's any point in considering legal action.
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". Would you buy from a shop that stated this? eBay is a jungle.
