The costume jewelry market is driven by trends and pop culture. One season, long necklaces for layering are in; next it's disco-era chokers. Some collectors bypass anything that isn't signed (designers stamp their name or logo on the reverse), so iconic pieces from noted designers and manufacturers (vintage Miriam Haskell, Kenneth Jay Lane, Weiss, Eisenberg, and others) command top dollar. But by nature, the bulk of costume jewelry is mass-produced, designed to bring a bit of glamour within the reach of everyone. That means there's an overabundance of pieces that, while pretty and intricate, fill the $5 or $10 tables at flea markets. And here, again, condition plays a part. It's hard to unload pieces with missing rhinestones or faulty clasps.
