There are different return policies. Sometimes resellers don't have the latest and greatest products. For example, some resellers are still selling new Macs with Panther preinstalled. However, in the U.S., you can usually find a reseller that won't charge you sales tax, and sometimes you can find good bundling deals. There are many reputable resellers, such as MacConnection and CDW, but some "resellers" are actually scams.
Apple Resellers are allot more flexible than an Apple store. They're are able to carry a more variety of 3rd party products as well as doing on-site house calls for computer repair and help. Most warranty and non-warranty repairs are done at the reseller store instead of sending the item to Apple. They're able to do upgrades to aging macs such as optical drives, hard drives,PCI cards, etc.
However return policies are much different and certain items(iPods) arent able to be fixed or returned at the reseller. Hope this answers your question.