I bought a few off of ebay, and didn't have a problem w/any of them. And "phoey" that a used pump can contaminate breastmilk. The ones pushing that are the makers of the pumps!!! The milk never touches anything that can't be washed! Also, your breast isn't exactly sterile when your baby nurses, right? But I digress...
The thing you need to ask when buying a used pump is when was it originally purchased from the store. I'd also ask how often it was used. Pumps don't last forever, but a decent one (Medela, Whittlestone, etc.) should last at least thru a few babies. Also, consider buying one new yourself...then selling it when you are finished. In the end, it will cost you about the same amount, AND you'll have the satisfaction of knowing that you got the best service out of the pump.
Congratulations on your baby, and I pat you on the back for nursing. I also nursed my son, and pumped for him for 9 months after I went back to work. Sometimes I felt like I nursed a breast pump, but all in all, I'm glad I made the sacrific to pump for him.