I would take into consideration where it is coming from. Do you know the person? About how much use has the pump already gotten? A friend of mine had to have her pump (electric Medela) repaired a few times between two children. If it is in great shape and you know the person it is coming from, how much use it has gotten, etc., then you might feel comfortable using it. I know that "they" strongly discourage using a used pump, but I have used friends' pumps and shared my breastmilk (CLOSE friends!). I used an Avent Isis manual pump full-time (pumping five days a week for almost 11 months) with my first, and the same pump lasted about 10 months full-time for my second son. The pump itself finally lost suction, so a co-worker loaned me hers (same pump) which I sterlized and used my parts on. I did NOT want to buy a new pump for two more months of pumping. I was not uncomfortable with this because I knew her, I sterilized it, and I could see that there was nothing trapped on it. So...maybe not too helpful, but some things to think about. Again, go with how well maintained it is, how much use it has gotten, and how well you know the person it is from. One more consideration--if you expect to be pumping full-time for several months or more, and/or for more than one child, you might consider investing in a new one just so you aren't stuck halfway through with a worn out used pump and facing buying a new one anyway. Good luck!
By the Way: As I said, I pumped full-time for almost 2 years (between two kids) with an Avent Isis Manual pump and LOVED IT!!! They are inexpensive, I found it easy to use, and very efficient. I borrowed a Medela electric and did not like it at all. I have several friends who loved their Medela pumps (as much as we are able to "love" those things!). Don't be frustrated if one kind does not work as well or as comfortably as another. :)