I don't see a problem using an old breast pump, just make sure you buy a new kit for it. After all, you can rent hospital grade breast pumps, they just make you buy your own parts. Even the multi thousand dollar hospital breast pumps used by the lactation consultants are used by many women, but not the tubing, flanges, and collecting bottles. Its not that you cannot sanitize the plastic parts that come in contact with milk, but the parts need to fit very tightly together to maintain the suction to work. Over time they stop fitting well and you won't get the strong suction necessary to extract milk. I just tried to reuse my kit from my second kid (now three years old) and there was a loose fitting part that prevented strong suction. I had to buy a new kit with each kid for this reason.
(fyi, i rented pumps for my first two, but a neighbor have me her old breast pump and I was very grateful not to have to spend $250 on one. I picked up a fresh Medella kit for $40 at Target)