Resellers on this forum and others will be buying during the holidays (most will be looking for deals), but as for end users I've personally found sales do drop this time of the year. Tons of different reasons I personally believe are behind this..
Taxes, end of the Business Quarter, and Bonuses - It's the end of the year. Companies need to start planning for taxes, reporting of Q4 earnings, and bonuses. Many companies will slow things down this time of the year to prepare for some or all of that. Once the new year hits they can start spending again but for now they need money to pay the tax man, bonuses to give employees for the year, and for those bigger companies they may want earnings to be high and costs to be low for investors (I'm guessing this type of thing would probably only apply to companies buying 6 or 7 figure type domains)
Holidays and Vacation - People travel for Thanksgiving and Christmas so it's the time of year allot aren't spending as much time on the PC or doing work. If you are emailing a ton of end users then some might not even see your email for a few days.
Unless you work in retail I think things are generally slower for everyone this time of year. Even regular office jobs slow down the pace this time of the year and it's probably got to do with some of the reasons above.