In my experience it is not uncommon for new exhausts to need some modifications or realignment – even if they are designated correctly for the make model and year. This is especially so with pattern parts, but sometimes happens with original ones as well – I suppose that the manufacturers assume these will be fitted by a garage in which case a minor twick would not be a problem.
A Ripspeed free-flow manifold I purchased some years ago had its tail section positioned at 30 degrees to the bit it was supposed to connect to – I had to cut-off the relevant section from an old pipe and bend it to create a new joint, and a brand new Abarth back-box needed its mounts modified to clear the car’s chassis…
As for comeback, in a theory a reseller is 100% liable, but in practice if you bought off eBay and the seller sold it in good faith, there is little point in perusing this. If there are any marking on the unit, you could try and look it up on the Internet to see if it is indeed the correct item for your car as confirmed by the seller, and if it turns-out to be the wrong P/N than you could say it was wrongly described.
Other than that – can it actually be fitted with some professional help, or is it totally wrong?