The upside of May is buyer momentum built over the previous months meeting a still-limited supply of active listings, which tends to support a stronger negotiating position than sellers see later in summer once more competition arrives.
The tradeoff is that a May listing needs to be genuinely ready. Buyers touring in spring have more options appearing weekly, so a home that is not staged, priced and photographed well can get passed over for a competing listing that is.
May's timing helps a seller who is prepared. It does very little for one who is not.
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