This depends on a variety of factors. A good time to consider selling is when market inventory is low and demand is surging, which typically results in higher prices. A good time to consider buying is the opposite; when there are more homes available than there are willing buyers, which ultimately pulls prices down.
If you're a buyer, you may have more negotiating power if a home has been on the market for awhile. Conversely if you are a seller, the more accurately your home is priced, the higher the likelihood that you will garner significant buyer interest, which puts you in the driver's seat.