thats what apple support says
You may install memory (RAM, VRAM), and other customer-installable parts without voiding your Apple warranty.
Apple's warranty states:
"This warranty does not apply...if the product has been modified without the written permission of Apple..."
Adding memory (DRAM, VRAM) or other user-installable upgrade or expansion products to an Apple computer is not considered a modification to that Apple product. Therefore, it is not necessary to obtain Apple's written permission to upgrade or expand an Apple computer. While Apple strongly recommends that you retain the services of an Apple Authorized Service Provider to perform any product upgrades or expansions, you will not void your Apple warranty if you choose to upgrade or expand your computer yourself. However, if in the course of adding an upgrade or expansion product to your computer, you damage your Apple computer (either through the installation of, or incompatibility of the upgrade or expansion product), Apple's warranty will not cover the cost of repair, or future related repairs.
so i would rather do it myself then risking someone at ebay to do it
as everybody can offer upgrades at ebay , if goes well for a while you get good feedback ...but
i could offer it too ,i have experience in pc upgrades ,and i know my macs inside out
and i have good feedback on ebay too
so i could call myself a mac specialist and offer upgrades , despite the fact that i never even have owned a iMac build after 2002 and offer upgrades on iMacs 27".. i even have 2 suction cups ...ok its actually the screen holders of my garmin and tomtom satnavs, but it might work, i could always use a screwdriver to wiggle out the glass ..so you get the idea ..do you want your iMac which is new to undergo such treatment ?
so i would be VERY VERY careful at ebay , ok there are some honest people out there on ebay who know what they are offering, but there are a lot of black sheeps too ,
and are they insured if they damage your imac ... removing the glass and drop broken, screwdriver dropped on the now unprotected lcd display ...will you get it replaced ..and when ?
at a apple repair center you are on the safe side if they break it they repair it and no warranty voided.. might be worth the extra money