You are correct, it is too long ago for you to report it to eBay.
If this was a one off sale the seller made, nothing more you can do, they themselves may have even unwittingly bought a fake item.
However, if this seller is still selling these items , slim chance but they may be, you could report back to Dyson giving username ID, then it's up to Dyson if they wish to notify eBay, and up to eBay to act on any information received.
@rugbyplayer6969