This article is from the Macintosh for sale FAQ, by Elliotte Rusty Harold firstname.lastname@example.org with numerous contributions by others.
Due to Apple's reluctance to authorize mail order dealers,
there are few authorized Apple mail-order dealers (unlike in the
PC world). Unauthorized "Gray Market" dealers take advantage of lower
foreign prices in countries like Mexico by buying computers there and
reselling them in the United States, sometimes selling them to the end
user for prices as low as other dealers can get their machines wholesale
from Apple. They also buy excess inventory from authorized dealers at
cost. The gray market dealers get product, and the authorized dealers
get steeper discounts from Apple for ordering more machines.