Quote 1
Then Bilbo fled [with the cup]. But the dragon did not wake – not yet – but shifted into other dreams of greed and violence, lying there in his stolen hall while the little hobbit toiled back up the long tunnel. His heart was beating and a more fevered shaking was in his legs than when he was going down, but still he clutched the cup, and his chief thought was: "I've done it! This will show them. 'More like a grocer than a burglar' indeed! Well, we'll hear no more of that." (12.17)
Bilbo's still trying to prove himself to the dwarves even now that he has gotten them all the way to the Lonely Mountain thanks to his wits and good luck. Obviously, that line in the first chapter that Bilbo looks "More like a grocer than a burglar" really smarts. Like the dwarves, Bilbo doesn't seem to be thinking of his quest in grand moral terms. Given that this quest doesn't seem to be about good vs. evil exactly, why do you think Gandalf has chosen to get involved?
Quote 2
"It will not be long now," thought Bilbo, "before the goblins win the Gate, and we are all slaughtered or driven down and captured. Really it is enough to make one weep, after all one has gone through. I would rather old Smaug had been left with all the wretched treasure, than that these vile creatures should get it, and poor old Bombur, and Balin and Fili and Kili and all the rest come to a bad end; and Bard too, and the Lake-men and the merry elves. Misery me! I have heard songs of many battles, and I have always understood that defeat may be glorious. It seems very uncomfortable, not to say distressing. I wish I was well out of it." (17.62)
Perhaps one reason why our hero Bilbo stays on the sidelines of the Battle of Five Armies is to show how un-glorious battle really is: unlike the people in the midst of the fighting, Bilbo has the space and time to realize that war is "very uncomfortable, not to say distressing." Even if his friends are fighting on the right side, that's not much comfort when he begins to think of the cost in lives.
Quote 3
"I beg of you," said Bilbo stammering and standing on one foot, "to accept this gift!" and he brought out a necklace of silver and pearls that Dain had given him at their parting.
"In what way have I earned such a gift, O hobbit?" said the king.
"Well, er, I thought, don't you know," said Bilbo rather confused, "that, er, some little return should be made for your, er, hospitality. I mean even a burglar has his feelings. I have drunk much of your wine and eaten much of your bread."
"I will take your gift, O Bilbo the Magnificent," said the king gravely. (18.46-9)
Bilbo's moral code is incredibly rigid when it comes to payment: if he has received a service or profited off somebody, he has to pay for it. It's not just the Elvenking; he also leaves the stolen keys with the drunken guard in the Elvenking's dungeons so the poor guy doesn't come in for too much criticism. Perhaps this strong moral understanding of the importance of payment is why Bilbo is so reluctant to accept much treasure?