I am buying a new MBA for my daughter...
Is there any good reason ethical or otherwise that i should spend an extra $200.00 and buy from Apple over other re sellers ?
If the retailer you purchase from is "Apple Authorized" they will prominently show that somewhere in their store (or, if online, on their Web site.) Purchasing from an Apple Authorized Retailer is just like purchasing directly from Apple. Apple should fully support you and honor the standard warranty on the product.
Apple has very cagey agreements with their resellers. While here in the U.S. it is illegal for for a manufacturer to tell a retailer what price they can sell their goods for (it's anti-competitive, otherwise known as price-fixing), what Apple does to get around this is they do not allow retailers to *advertise* lower prices than other retailers. So, having a sale is almost a waste of time, since consumers generally won't know the sale exists. This is all done in an attempt to keep prices for all Apple retailers high, and thus it allows all of them to make more money (assuming Apple also keeps demand high by not authorizing too many retailers for a given geographic area). This is technically known as "protecting your market".
Some authorized resellers get around not being able to advertise low prices by including extra products with the sale, or by giving rebates, and advertising those things, rather than a low price. So it's a bit of a dance. But the bottom line is that you aren't likely to find genuine Apple products, being sold by an Apple Authorized dealer, at an unusually large discount. It may be different in different countries.
One thing to avoid is "grey market goods." These are genuine Apple products sold by non-Apple Authorized retailers. Apple has agreements with their authorized retailers not to sell Apple's products at wholesale to non-Apple Authorized retailers. But some authorized resellers do so anyway, because they have more stock than they can sell (often because they purchased too much stock from Apple to get a volume discount). Authorized dealers selling to non-authorized dealers is known as "trans-shipping" and doing this is a violation of the contract between Apple and their Authorized dealers, and is illegal. Selling grey market goods also keeps the consumer from being "in privity of contract" with Apple. (That is, the consumer who purchases grey market goods has no legal relationship with Apple. Their relationship with Apple is precluded because Apple has no relationship with the grey market vendor.) In the U.S., a manufacturer (e.g. Apple) may or may not honor their usual warranty on grey market goods. So, if you see an Apple product with an unusual discount, you should immediately be suspicious that it is grey market. Once again, this may be different in different countries.
(NOTE: There have been legal cases in the U.S. where consumers have gone to court and they have prevailed, forcing a manufacturer to honor the usual warranty on their products, even though the product was sold through the grey market. However, going to court to enforce a product's warranty is almost always something that you want to avoid having to do. So purchasing grey market goods may not be a good deal.)