I can relate man. It's the biggest reason I no longer deal on EBay.
I left with 100% positive feedback but the point being, someone screws you, you risk your own good name there when you call them out with negative feedback as they can simply leave negative for you.
I hope the dude follows through with you in either returning the pump or paying you. Simply put, it is far more embarassing to be called out publicly than it is to just say Hey...I can't afford this right now and I am sorry for the trouble. At least then you seem like a stand up person who is willing to admit when they are wrong.
That being said...I still haven't found anyone to loan me $5 for a package of depends...