I second the advice to buy it NOW and leave the tags on. It was pure misery trying to find a white dress in November, and I live in a huge city with tons of shopping.
If you get a bid, it would be a good idea to ask that first week what kind of dress to look for (what the requirements are for length, sleeves or no, etc.).
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"A child of five would understand this. Send someone to fetch a child of five."
Groucho Marx