Shemiras Halashon Halachos

Hilchos Lashon Hara chapter 5, section 7

Just as one may not speak against a person, so one may not speak against his possessions. It is very common for shopkeepers to disparage another shopkeeper's goods because of envy.

Hilchos Lashon Hara chapter 5, section 8

It is forbidden for even one person to disparage someone and how much more for two people to disparage someone. The latter case is worse because people are more inclined to believe a report said by two people.