Strategy

It's a bit of a game, figuring out when to buy something full price and when to wait for a sale. You have to assess how popular you think this item will be, how much you love it, how much you're willing to risk not having it. Sometimes you win, and sometimes you lose.

Certain designers in certain designs from certain stores will sell out faster.

I've noticed a few things:
. boutiques sell out much quicker. they probably make smaller orders.
. trendier items will sell out quicker as well. but they have to be wearable trendy items. so the Isabel Marant pinafore dress sold out immediately, while the Vanessa Bruno bubble mini skirt did not.
. obscurish, indie, or edgy designers may sell out quickly, depending. truly edgy stuff will not. cute designs from indie designers will.
. every season there are "it" items. those you have to get on a wait list for and hope. definitely no waiting for a sale.