These are the letters which Endymion wrote
To one he loved in secret and apart,
And now the brawlers of the auction-mart
Bargain and bid for each tear-blotted note,
Wilde here is referring Keats to Endymion, the hero of Keats’s poem  Endymion . The  opening line of this poem  can describe Keats’s letters quite well: 
 A thing of beauty is a joy for ever:
Sonnet: On the Sale by Auction of Keats’ Love Letters