Sure is used to signal consent, or to put it another way, willingness to go along with something. It's often used in response to requests for permission:
Alice: Would you mind if I take the car?
Bob: Sure, go ahead.
In the above, Bob is willing to go along with the proposition of letting Alice use his car. Bob is not signalling literal agreement with Alice's words, which is exactly what yes does:
Alice: Would you mind if I take the car?
Bob: Yes, I would mind very much. Take the bus.
Compare the following:
Alice: Would you mind if I take the car?
Bob: No, go right ahead.
In this case, literal disagreement has the same illocutionary force as sure, so we can see that yes and sure don't mean the same thing. However, the opposite might be true in another situation:
Alice: Would you like some cake?
Bob: Yes, I'd love some.
Alice: Would you like some cake?
Bob: Sure, thank you.
Here, Bob's willingness to go along with Alice's proposition has the same force as literal agreement. No would signal the opposite:
Alice: Would you like some cake?
Bob: No, thank you.
In other words, sure signals consent, while yes and no signal literal agreement or disagreement.
It does indeed mean no, and this isn't even just "Hagrid-speech"; nah is a common informal way to say "no".
I'm not sure where you mean it should have been placed in the previous sentence, but I'll attempt to explain why it appears where it does.
They didn' keep their gold in the house, boy! Nah, (the) first stop fer us is Gringotts.
There's an implication there in the nah that's based on prior context."The gold isn't in the house; there's no reason to go to the house. No, that's not where we need to go; the first stop for us is Gringotts!"
Basically in this case, the nah is being used to say "No, not that thing you just said. We're doing this." Another example:
Amy: "Did you try that new Italian place on your date last night?"
Mary: "What, and completely ruin my diet? Nah, Mark took me to a sushi place instead."
Best Answer
Timber is wood in its aspect as a material for making things: building houses and crafting furniture. Ordinarily we want straight timber for such things; imagine trying to build, say, a set of bookshelves out of something like this:
(image from http://www.woodmagazine.com/woodworking-tips/techniques/skills/dealing-with-wood-defects1/)
So Kant is saying that humanity is so irregular that you cannot make anything perfectly straight from it.
Note, however, that this is a translation from a German original whose nuance is unusually difficult to express. There are interesting discussions here and here, which may help you understand not only the sentence itself but the great range of considerations which go into translating even so simple a sentence as this.
Here is my own translation, somewhat 'free', of the passage in question: