There’s a sefarim store that always treats me nicely, as opposed to this stores competitor. There are a number of stores that are cometitors. Can I compliment the nice store by saying, “Your always so nice to me unlike your competitors.” I”m emphasizing the fact that the other stores are rude. Would this be lashon hara? What if other customers are listening? I might be mazik somebodies bussiness?
You should compliment the store by thanking them for being so nice, without mentioning the competitors. Mentioning the rudeness of other stores is lashon hara, and also unnecessary. However, it might be worthwhile going to the other stores and telling them that you are taking your custom to a different store, where the service is more polite. This could encourage them to give proper service.