I think it's great that you want to be "green" about your pump. However, unless it is hospital grade it's acutally not sanitary to use someone elses pump. I know that there are plenty of people that are selling/buying used pumps but the motor would actually need to be replaced which isn't really cost effective, (it sounds weird I know, but check it out : Personal use pumps should never be resold or shared among mothers. The Medela Pump In Style® Advanced has an internal diaphragm that cannot be removed, replaced, or fully sterilized. Therefore, the risk of cross-contamination associated with re-using a previously owned pump such as the Pump In Style cannot be dismissed, even when using a new kit or tubing.)
http://www.medela.com/NewFiles/preownpump.html This is per medela's site.
Not to mention if you speak to any lactation consultant they will recommend NOT to use someone elses pump. Essentially it's like using someone elses toothbrush. (Gross!) So personally if it were me I would not sell or buy a used pump.
BTW, we're not necessarily talking about infectious diseases however bacteria.