Startled at the stillness broken by reply so aptly spoken,
"Doubtless," said I, "what it utters is its only stock and store,
Caught from some unhappy master, whom unmerciful disaster
Followed fast and followed faster, till his songs one burden bore,---
Till the dirges of his hope that melancholy burden bore
In the prior stanza, the narrator said that the raven, like hope, will leave him. The raven says "Nevermore".
So to reassure himself, the narrator says that the raven could not possibly have replied to him, that what the raven says is just something an old master taught him (stock and store, meaning 'a stock reply' or a generic answer).. The former master was unhappy and thus spoke only 'nevermore'.