Advantages:
Rummage sales and similar events such as bazaars, are usually sponsored by a religious institution like a church or synagogue or by service organizations such as the animal clubs—Lions, Elks, Moose, or a group like the Junior League.

They usually involve large numbers of people who collectively bring their items to be sold on a pre-set day and location (usually indoors) after which the proceeds go to the organization on whose behalf the event was held.

In some ways, these rummage sales can be a lot like having many yard sales in one location. There are usually lots of treasures, and because they have been donated, they are generally well-priced so deals are plentiful.

There is often a table with homemade goodies (cakes, jams and jellies), or hot soup or other refreshments adding to the shopping ambience.

Downside:
One of the only downsides to rummage sales despite the obvious lack of predictability (unless they are annual or otherwise regular events with a particular reputation) is their general lack of frequency. You usually have a somewhat limited number of these in most communities.

Bottom Line: It's worth looking for rummage sales in your area. You can get a great deal and contribute to worthy causes.