A lot of have gotten a used pump somehow. Maybe your sister gave you her old one, maybe a friend passed one on - it happens all the time. Is it OK to do? Only you can decide, but here are a few facts to help you make that choice.
(PIS = Medela Pump In Style, PY = Ameda Purely Yours)

Motor Power

Most electric breastpumps are designed to be used for one year. This means one year of full-time use (equal to about 15 pumping sessions per week). Will they last longer? Some do, some don't - it depends on how the previous owner used it. But if you have a used pump and the suction seems to be decreasing, it probably is. You can replace all the valves and membranes, but beyond that, there's not much you can do. And, of course, the warrantee is voided if you're not the person who bought it.

If you know the previous owner only used it a couple times, you can expect to get more life out of the pump. (And I'll hope that she didn't use it because she chose to stay home, instead of because she decided not to breastfeed.) But - before you decide to use it, read the next section about germs and contamination.

Cleanliness

An electric breastpump is designated as a "personal" item - this means that it is not built for sharing, and there's no guarantee there won't be any contamination when there are multiple users. (Of course, this doesn't apply to rental pumps, which are designed for multiple users over many years).

What this really means depends on the design of the pump. Is the milk pumped into a sealed chamber, or can it go back up the tubing? If milk could possibly go back up the tubing (the PIS is like this), there is a chance a used pump will be contaminated from the previous users milk. If this person is your sister or a close friend, this may not bother you. But this is why I would NEVER use a pump from someone I didn't know personally. Ew. And at the very least, I would take the cover off the motor housing to make sure it didn't look like there had ever been any milk in there. I've heard stories from Lactation Consultants popping the cover off a PIS pump and finding actual mold. Again, ew.

Some pumps pull the milk into a sealed chamber as it goes into the bottle. With this kind of pump, there is not a way for the milk to creep up the tubing and potentially contaminate the motor. The PY has this kind of design, so I would be a lot more comfortable using someone else's PY than the PIS.

My Conclusions

I am under a certain ethical obligation to say that I think using someone else's pump is a bad idea. But - I know the reality - these things are expensive, and if a good friend has a barely used pump that she's willing to hand over to you, it's hard to say no! Once you've read the material above, I think you're qualified to make your own decision. At a minimum, I would replace the flanges, tubing, and all of the soft rubber parts. And keep an eye on an older pump for signs that the suction is decreasing. This should be your first suspect if your supply starts to decrease.

Personally, I would use a pump from a friend (I did this when I first went back to work - until the motor died after 4 months, when I'd saved up cash for a new one), but I would NEVER use a pump from someone I didn't know. And buying a used pump off eBay? Totally ew. It just skeeves me out to think about it. Who knows what people have been doing with that thing! If you're pinched for cash, there are plenty of brand new unopened pumps on eBay - look for the abbreviation NIB - which means "new in box" if that's the route you choose to go. There are also a lot that say "used once" or "barely used" - that trips my skeevometer.