Is it violation if a seller use other platform name in backend search term?

It might cause issue for you, so make sure that you avoid doing so for your backend search terms.

Although you are using it as backend search term, but you should avoid to use the name of another platform as it might cause issues for you,

Using the names of well-known platforms like Walmart or Target as backend search terms could potentially create legal issues, especially if it results in consumer confusion or suggests an unauthorized affiliation or endorsement. This kind of action may infringe upon trademarks, which are protected intellectual property rights.