Not necessarily; people do purchase certain horses in the winter - if you have a hunter or a jumper then lots are looking to compete during the winter season.
Dito Samp i buy sell winterish (tend to alittle before just so i can give them more fuss before the dark winter nights *Need know hes not going to kill me before I do things in dark(!)* This way horse is well settled and and bond can form before the showing season etc. Also time to play about with him get him right. A good horse will sell anytime and more importantly a good home will buy anytime!
I agree with everything that has been said, apart from the reply about waiting till after xmas.... good horses sell and good owners will buy no matter what time of the year - look at us - we've just bought again and that means no xmas pressies for the next 20 years I'm afraid, not sure if birthdays are out too, but hey who cares!
After christmas, when the nights are still long and dreary. everyone is fed up of being cold, wet and miserable, and can't really be bothered to go and muck out...again.
Sold my boy 2 weeks ago to the first viewer - vetted and gone within 7 days. Got my new one now too. If someone wants what you are selling then there you go....