Is it permissible to sell used water filter parts to people suspected of repackaging and selling them as new?

It is permissible to sell used parts to these people, as long as the matter does not go beyond mere suspicion, and you do not know how they will use them. There is no doubt in dealing with a discreet Muslim, and whoever refrains from doing so out of scrupulousness has innovated. All money that is not known to be usurped or acquired in an unlawful manner is permissible to deal with.