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Bringing a bunch of random items to a low-traffic dungeon and waiting in hopes SOMEONE shows up and MAYBE buys your stuff hardly seems like a profitable endeavor and there's no way Kecleon would waste time with that over setting up shop in high-traffic areas like Treasure Town.
Their REAL business in dungeons is taking the items and money of explorers who faint, the crappy little shops in there are just a cover story so nobody gets suspicuous. Did you really think the wild Pokémon are the ones digging through your coinpurse every time you wipe out?
I've yet to see a convincing rebuttal to this.