Those music shop bellows on the counter are a big con. All they tell you is that the harp has reeds inside it. It can't tell you whether the reeds have evenness of response (always a massive issue with cheap harps) or whether they are properly gapped, and you won't pick up much out-of-tuneness either. The reason? They do not play the harmonica with anything remotely resembling a human embouchure, and embouchure is everything (almost). I gave up buying super-cheapie harps a long time ago. Every harp I buy these days is OK off the shelf, though there may be minor gapping issues to fix (usually because I like gaps slightly more open than they sometimes come). Any harp that isn't fine-tuned to equal temperament gets retuned before I take it out to play. I rarely have to do any corrective work at all on Suzuki Bluesmasters or Promasters, and very little on Lee Oskars (they can come somewhat narrowly-gapped for my taste). Hohners are never quite right for me out of the box, but the required tweaking is easy and quick enough to do.

As for buying second-hand, well you can't live your life thinking that everyone in the world has a deadly transmissible disease. Having said that, I wouldn't buy a second-hand harp with a wood comb or one with valves, both of which are gunge and germ traps (you can, of course, if you can be arsed, change all the valves). But I'd happily buy a harp that is all plastic and metal which is unvalved. I remove the covers (and mouthpiece, if applicable) for a good clean, wash the assembled innards in warm soapy water, scraping off any gunge, dry and give a good wipe with isopropyl alcohol. A far bigger issue is whether the thing is being sold because it's knackered!