The Blood of a Dragon — Lawrence Watt-Evans

It wa definite -- Dumery had no magical talent at all. He could never fulfill his dream to become a wizard's apprentice. Hurt, angry, and mortally disappointed, he despaired of ever choosing a trade.
But then Dumery spied a so-called great wizard humbling himself before a man selling dragon's blood -- the precious stuff that made difficult spells work. If Dumery couldn't be a wizard, maybe he could become a dragon-hunter and have all those scornful wizards crawling to him.
So, leaving his family, city, and comfortable home, Dumery began trailing Kensher, the man in brown -- even though Kensher said he didn't need a dragon-hunting apprentice. But when Dumery finally caught up with Kensher, he would discover Kensher's great secret of how the precious fluid was obtained -- a secret from which only Kensher's kin could profit.
Once again, Dumery would be left without career or future. Unless . . .