Its is always good to buy certified diamond, but they cost little more. You can sell in decent price if it needed. Never buy diamon from chain stores like Kay, Zales or whatever if this cost more than $1000. Any diamond less than $1000 is just a waste of money. Rather, buy 24ct gold, if your soon to be wife like gold.
As said by usrfocl, blunile is good since they send you GIA certified diamond.