I did a bit of research on this a year ago as I was given my aunt's medela pump and contemplated the same as you. Here is what I found and what I did:
YES LADIES THERE IS A RISK: THE AMOUNT OF RISK DEPENDS ON YOUR SITUATION!!!!!!!
Tech. speaking, hospital-grade pumps can be fully taken appart and sanitized where as individual pumps can not, making the internal parts not able to be sanitized fully. This can possibly cause concern because breast milk can harbor disease and infection, and though the internal parts are not supposed to come in contact with the actual fluids in milk, the disease/infection can travel through the moist air that enters through the tubing AND, if the pump for some reason backs up, it can "spill" into the interior parts causing contamination! Plus, the research says it should never be used by a second person because these deseases/infections can go un-diagnosed/un-found in mothers and babies making it hard to know if the first user was a carier.
No maker of personal pumps will honor warranties of pumps used by anyone other than the original buyer. Also, no lactation consultant will ever recomend you use one other than new either.
When I asked several consultants about it, they all advised against it, even with it being my aunts, but a couple did act like it was a bit "more" safe since it was a family member than a coleague or stranger.
As I did further research, I read from a reliable source (though I don't remember the name of the source at this time-sorry) that sharing a pump was like sharing a tooth brush. It said if you would't share a tooth brush with that person, don't share a pump with them either.
After looking into the research and talking with the consultants: though they never SAID OK, prob. b/c of legality (didn't want to risk being sued should anything happen), it was like they could see it being ok in my situation (could tell from body language). These consultants (the ones I talked to) never make a dime off of sales (as someone said they think it was all about sales, and that may be from corporate like Madela, but not here), so that did not influence their "decission", but they still wouldn't commit to an "OK". I did decide though that it would be fine for me to do so. After all, I WOULD use my aunt's toothbrush in a bind and after some (even if minute)cleaning/disinfecting it in hot, boiling water, as best as I could. And, though possible, I highly doubted she had any disease that would put me or my babies (2 from 2 pregs.)in harm.
Well, I got all new parts and starilized as much as I could on the pump and parts(I took the diaphram cover off and "cleaned" the diaphram and anything else I could get to and sterilized the new parts b/c you don't know what they come in contact with in the making of them). I'm on my second baby and both are very healthy! I figured my decision was better than formula no matter what and as long as I was logical about where the pump came from. For me, the risk (none to very little) outweighed the benefit (free, breast is best, etc).
I say as for you, the choice is totally yours: IS IT WORTH THE RISK? In my case, I saw no or very little risk. In your case, do you see any risks? Would you use her toothbrush? Would you let your baby(ies) use her toothbrush? Simply buying all new accessories does not eliminate the risks, just lowers them. People do use others pumps all the time (way more than should probably), most with no known probs. You are at an advantage over buying/using from someone you don't know or don't know all that well, but there is a (even slight) risk! I'm not saying don't do it (as I did use my aunts) and not saying do it either, just know the health history and the person well from whom you recieved the pump and contemplate does the benefit outweigh the risk? For me, it did!
Best of luck!
T.