I am a seller on Gunbroker.
There are definitely times when it's busier and slower.
When it's busier, prices are higher (more bidders). Of course, when it's slower, prices are down and you can get better deals.
Seasonal peaks and valleys:
Mid-Sept thru Oct - very busy as people get ready for hunting season.
Nov until Thanksgiving - slower time due to upcoming holiday
Thanksgiving until week before Christmas - very busy, holiday gifts to self and others.
End of Dec until mid-Feb - very slow due to credit card shock from Xmas buying.
Mid-Feb until near Easter - Busy due to income tax refunds
Easter to Memorial Day - Slow period, tax season, etc.
Memorial day to July 4th - Busy due to preparing for summer shooting/camping, vacations, etc.
July and August - Dog days with people on vacation and heat keeping people indoors.
If you want the best prices then buy during the slow periods. If it's an actual auction instead of a fixed price auction chances are you may be the only bidder. Right now is prime time to purchase a firearm on Gunbroker since we are in a heat wave and it's post 4th of July. Sales have been very soft for the last few weeks. Prices rise when the air gets crisp and thoughts of hunting season start to occur.