Are you allowed to work at a store (pet shop, for example) that does tzaar-baalei-chayyim to the animals it has there?
If this is a Jewish-owned store, and working for them facilitates the sin, it is probably prohibited to work there.
BUT You should find out if the Tza'ar being committed in the store is in fact halachically impermissible. There are many valid situations where the tza'ar is permitted.
If this is a non-Jewish store, I don't know of a source that prohibits tza'ar balei chaim to non-Jews, so there's no Lifnei Iver. Of course, if the behavior is illegal according to American law, that would be entirely different.