Is it ethical..?

Legally there would be a reasonable expectation of behaviour. Any attempt to use or profit from those goods would breach that expectation. Reasonable expectation would also imply a fair time frame in which an effort to return the items, or at least notifiy the legal owner of receipt, should be made. A grey area, but once you are outside of the bounds of reasonable your defense diminishes significantly.

There you go. That said, what La Bendix said about intending to send them back and never getting round to it and feeling guilty for years sounds perfectly reasonable to me.