In Canada, where I live, interest in vintage bicycles drops like a rock in mid November and starts to pick up again in sometime in mid January. That was when the markets were strong, which most no longer are. More important than time of year is location of sale, what is being sold and the seller's marketing skills. To get top dollar, the seller must also be patient, waiting until the right buyer comes along.

Perhaps Basic Trading or On-Line Trading will shed a bit more light on how to make more with what you have to work with.