Pumping can be stressful enough without worrying about whether your pump is working correctly! At Good Buy Gear, we check that the electronics work, but we don't actually test breast pumps out on ourselves, so we can not guarantee the effectiveness. We do ask sellers to only consign pumps that are still effective and strong.
We also recommend purchasing a closed system pump (e.g. Spectras) as it is not possible to completely sterilize the inside of open system pumps (e.g. non-hospital grade Medelas) without completely disassembling them. The risk that your baby could become sick from the contamination in your pump is low but a risk nonetheless.
If you are unsure, we recommend visiting our friends over at The Mamahood and attending one of their wonderful breastfeeding classes.