We bought a bumbo, swing, tub, and playmat from Craigslist, washed the cloth parts in the washer, used Lysol wipes in the rest, rinsed the toys off afterwards.
I thought the same thing about used pumps, but a friend told me with many kinds, you can buy replacement parts inexpensively (tubes, etc) so its not like the 'leftovers' from the previous mom would still be in there (ewww). Maybe call the manufacturer of the one you're looking to buy 2nd hand to see if this is possible and what the cost would be to see if it's worth it.