I understand your point. What I was considering is that what if they are telling the truth, that the manufacturer provided the pump to them this way, for whatever reason. You call and tell them that you want your money back. What you've done is returned to them a pump that they may not be able to sell because of what the manufacturer did.
Assuming that, I can see why they'd be somewhat less than cheerful about it.
Now, don't get me wrong, it's not your fault that the manufacturer did this, and what the vendor should be cheerful about is being given the opportunity to make it right for your and maintain their reputation in the industry. What I am saying is that assuming that the stories on both sides are correct, then the vendor is being caught in the middle here, and at the end of the day, they are eating the loss and making it right for you. Reluctant or not, they gave you what you wanted, and may very well be taking a loss on the pump. To me, they deserve at least some credit for that.
Also consider that they probably get stories like that all the time from customers who are lying to them about it. They probably have people unbox the stuff, tear it all up, and try to return. Granted, you were being honest in this case, but in the end, it's probably something they've been burned on before, yet they did take your word for it and take it back. I've seen many vendors who say "Once it's opened, you're stuck with it," yet when they send you something in this shape, they say "No, you must have opened it, we wouldn't send it to you like that." Again, I just think they deserve a little credit for giving you the benefit of the doubt and making it right for you.
...oh, and on your side... if someone else did what I described above and return it used, only to have the Depot turn around and try to pass it off on you... then they should be horse whipped. Of course all this is moot because we don't know what that pump's story really is.