surely not fit for purpose ?
Probably not, but this has nothing to do with eBay. eBay has no further interest after their own 30 day money back guarantee has expired. After that, any responsibility rests with your seller. However, as you have discovered, getting after-sales service from many eBay sellers is difficult or impossible.
I don't knw what your phone is worth currently, but you've had it for two years and the original owner probably had it for at least as long if they replaced the battery with a fake aftermarket product.
My guess is that you will probably have to replace the battery at your own expense, if it's worth it, or bin it if it isn't. To be fair, eBay does warn buyers that they don't guarantee that items will be truthfully described, or even be safe or legal. Would you have bought a phone from a shop that stated this? Buying from unknown sellers oneBay is always risky.