I would buy a second hand closed system pump, but not an open one. Most medela ones, apart from the hospital grade, are open systems, so I wouldn't buy one of those second hand personally, as you can't get the whole thing clean. I found a bargain ardo calypso in the boots clearance a week ago, and I'd happily sell it on used as it's a closed system. I wouldn't want to sell an open system second hand either, because of contamination risk.
https://www.breastpumpsdirect.com/open_system_breastpumps_vs_closed_system_breastpumps_a/147.htm