In all cases, controversy exists to whether the differences are due to speciation or to domestication. The two may look alike, but they are vastly different. Behaviorally speking there are some major differences. While there exists a commonality of behavioral expression, the degree of that expression is different. Domestic ferrets are gregarious, polecats are solitary.
Ferrets tend to be more juvenile in behavior compared to polecats.
Most differences are not type of behavior, but degree of expression. Reproductively speaking they are very similar. While it is true that European polecats and domestic ferrets can interbreed forming viable offspring, that alone does not prove that they are the same species. The subspecies name, furo, actually means thief in Latin. Domestic ferrets can be litter box trained and can learn to do several tricks, depending on the patience of the owner.