For the first question, you need to go to Advanced Search (right below the search box) and enter all the things it needs to know (like this http://www.brickshelf.com/gallery/riofo
cklink.jpg ). Then, you search and it comes up and you visit the seller's store.
For the second question, it's hard to tell. If they have all the bricks you need, and it's only a few cents difference from the other store, use the one store so you don't have to pay another shipping charge. Minimum shipping is about 2 dollars, so the fewer stores you use the better.
I've never bought bricks from Bricklink like you are, I went to the LEGO store to get my last batch of basic bricks. So I can't help you much on the third question. You can ask them if the bricks are cracked, beat up, faded, or if they came from a smoke/pet free home. They can tell you, and all you can do it hope they they are telling the truth. Also, visit the buyer's Splash page to see if they charge extra handling fees, and see if they mention the condition of the bricks.
For basic bricks, I'm not so sure Bricklink is the way to go. Bricks can often look cheap on Bricklink, but when you get mass quantities of bricks, the price is raised. For example, on that search I showed a picture of earlier, the cheapest seller was selling black 2x4s at 13cents each. For 100 of them, you have to pay 13 dollars. However, if you go to the LEGO Shop website, they are selling 650 bricks for 30 dollars. That is .$0.046 cents per brick. A much more reasonable price. And if you have a LEGO Store nearby, you can use a Pick-A-Brick packing technique (like the one I used when I went to the LEGO Store), you can get bricks for about the same price. I paid 35 dollars for 629 bricks (not counting a half a bucket of transparent studs).
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