I've gotten a fair number of harmonicas used. A thorough clean in a diluted hydrogen peroxide/alcohol bath was all it took to set my mind at rest. There are lots of people who buy harmonicas thinking they'll play them, but give it up within days or weeks--their harps are no worse than those bought new, and cost far less for the same quality. We put all sorts of nasty things in our mouths (frequently without knowing it--check the allowable levels of rat droppings and such in foods), so worrying about what you might contract from a used harp after cleaning is just indulged hysteria.

Buying any harp is a bit of a crap shoot (intonation and play-wise) unless you learn to adjust them yourself or spend as much again to have an expert set them up for you. On this score, lightly used is as good as new. On the other hand, reeds do wear and become stressed, detuning them beyond feasible adjustment, so it helps to know the age and type of usage a harp has had.