I wonder if they are hacked accounts, or accounts that are set up expressly for the play? I don't have much of a criminal mind, so I don't know how it would work. Obviously, I bet there is no physical Alembic to buy, so you PayPal the fake 'Reverb Shop' money, and they obviously can't send you what they don't have... so howzit go? They send you a greeting card congratulating you on getting dorked out of $5K? You file a fraud case with PayPal and fight it out? That MK bass is listed as "Sold", not "No Longer Available". So somebody is probably about to be having a bad day.
Looks like Reverb would have some interest in putting the kibosh on this kind of thing. It's got to be costing them credibility. I guess it's so rampant now they can't police it all.