This article is from the Macintosh for sale FAQ, by Elliotte Rusty Harold email@example.com with numerous contributions by others.
Superstores like CompUSA are fairly hassle-free since they
won't do any haggling except on the largest orders. They should be
able to match a competitor's advertised price though you may need
to show them an ad with the better price first. On the other hand
any authorized dealer should almost certainly be able to beat a
superstore price. After all, noone puts their best price in an ad
for all their competitors to see. The best use for a superstore is
to tell salespeople at authorized dealers what prices they need to
beat to get your business.