Medela has info. on their website that says pumps are personal use items like a toothbrush and should not be used by anyone else other than the original owner. Some moms do it anyway.... but I personally would steer clear of any used pump and try to make the investment in a new quality pump if you can. Medela is a good brand but the type of pump you need will depend on how often you are intending to use it. If you are going to be workign full time you may need a better quality pump like the pump-in-style... check out the Medela website if you have some time I had found helpful information there back when I was getting started.
If you are looking for a used pump the Ameda brand (Ameda Purely Yours) are approved for multiple users. I just got one on eBay for $130 - new it is ~$300 because it comes with a backpack/cooler/bottles. I think it works great. My LLL leader and my doula both recommended Medela or Ameda pumps.
Try www.bestbuybaby.com for discounted pumps. I got the Medela PIS for $195 (retails for $230+). No tax if you are outside of NJ & no shipping charge.