Players are sold throughout the entire day. Your best bet for selection is when the most people on your server are awake.

Keep in mind if you are looking for "gens" they tend to be created at the beginning of the day on the old style TL, and when there are few players on the Market; and as Dman said they seem to be at a particular point in the hour on the new TL, but since the auctions times are shorter in the new TL, so too are the "gens" times very short.