I find you want to follow the patterns of the department stores. You primarily want to list your items about 1 week after they show up in the stores and before they start going on sale; if the stores are putting out great sales on new clothes used clothes are less apt to sell for top dollar.
Using that as a rule of thumb I found I do best if I listed the kids long pants/shirts items the first week of August as people want them for when school starts. Winter Jackets, Snow Pants, Gloves, Hats, Snow Boots are best listed in Late September/Early October.