If they aren't too careful what used CDs they buy, they may not be that great of a store. Ask if the store guarantees their used CDs. Don't be afraid to search through those 'bargain bins'. Often we put items that we have duplicates of in the bargain bin. If you hear of a new re-release of a certain artist or album, you may want to start checking the used bins the week that new release comes out.
Collectors strive to have every release by an artist.
If the new release has an extra track on it, they'll get rid of the old version and buy the new. Independent stores are also good for new music as well. Some, like us, have a best buy within 2 miles and need to stay competitive. A smaller store can often give you a bit of guidance and are much happier to see you walk in the door. And they can watch the incoming used for items you are looking for.