It is permissible to inform friends and neighbors of a store where they can obtain items of better quality or at better prices than at the store where they usually shop.
Here too certain conditions have to be met:
One should know the information firsthand. (If one cannot verify the information firsthand, he would have to say, “I have heard, but have not verified that…” He must be certain that the quality of the other merchandise is superior (brand names are not necessarily better); where there is a significant difference in price, one must verify that the less expensive item is not inferior in quality to the more expensive one.
There must also be no suspicion that the preferred store is dealing in stolen merchandise, as Halachah prohibits purchasing stolen goods.
In conveying the information, it is important not to imply that the first storekeeper is overcharging or is guilty of selling inferior products; one must convey the pertinent information without being judgmental. Furthermore, since people often view high pricing in a negative way, the information may be considered derogatory and could only be conveyed for a constructive purpose. Consequently, one may only relate the information to people who may be interested in making a purchase. Discussing the matter for the sake of making conversation might constutute loshon hora.
Finally, one should consider why he deems it necessary to relate the information altogether. Perhaps it would be better not to get involved and refrain from any action which would affect the storekeeper’s income.
Sefer Chofetz Chaim, 14 Kislev, Pages 196 and 408