My understanding is that commercial establishments, at least, can't legally sell a harp that hs been played. That's why stores have those bellows thingies, and will properly insist on your buying one if you play it with your mouth.

Private persons, however, may be able to avoid legal repercussions.

I have inherited a few over the years, and bought a couple on eBay (vintage ones), but typically use the thorough-cleaning system mentioned above. I typically use alcohol (vodka or gin, the cheaper the better) in place of soap and water, figuring that it'll dissolve crud better.

Bob