It's a trade-off.
If you like vintage equipment, used is pretty much the only way it can be obtained. Think about it. If a seller lists an enema bag or some such as NIB, the odds are that's not true. Very few people buy a bag ‘just t have one’. Bags are bought because someone needs it, and if it's a fountain syringe it was used for a douche or enema. Found in an old drugstore is very, very rare.
That said, if you buy a used bag or bulb, assume it has been used. That has always been by assumption.
So, when it arrives, it MUST be cleaned and decontaminated. This is generally what I do.
The box gets thoroughly wiped, both inside and outside, with disinfecting wipes, like the Chloro ones , left for a few minutes, then wiped again with a fresh wipe, and left to dry. If the box is very dusty, dry wipe it before using the wipes.
The bag, hose, connectors, and nozzles are completely disassembled and the metal clamp removed from the hose. Everything is then put in a basin of very warm water with some dish washing detergent and left to soak for a half hour or so. Be sure the bag and hose are full of the solution and run solution through the hose a couple of times. Do not use scalding hot water or the rubber of the bag may “bloom”….. develop a whitish surface film.
Rinse everything then repeat the above procedure using a bleach solution, about a half cup to a couple of gallons of water. Remember to remove the metal shutoff and any other metal parts from the bath, else they may corrode. Again, no scalding hot water. Let all the non-metal parts soak for a half hour or more. Fill and re-position all parts several times during the soak
This treatment should kill pretty much anything organic that hasn't already died from sitting somewhere for many decades. Another option is to use something like Wavicide in place of bleach.
YMMV