Well there are VERY few pumps that are a closed system. Ameda makes one, lanisoh has one and a few medela ones are... The medela ones are hospital grade ones.
The rest are open systems which means the milk goes through the entire pump. Just to remind you HIV virus can be transported in breast milk and you can't clean an open system. Milk is in the inside and short of taking it completely apart and find someway to sterilize every part inside it's still in there. There are lots of diseases that transfer in milk to child. Medicines of course. All sorts of things. I had one friend that decided she'd somehow get bleach run through the inside... All I could think was yeah bleach is better... But that was her decision I know.
So if you do want to use someone elses you need to get one of the few closed systems like Ameda, Lanisoh or a hospital grade Medela. Otherwise you are being unsanitary plain and simple. If you do get those you need to still replace all the flanges, valves, piping etc. They sell them as a kit for all those pumps by the way.