Okay, but does Ushigi-san actually, you know, pay? Because ¥6,660,823 is a bit over $40,000 USD (which is about 480,000 Norwegian krone), so unless she's a real old money trust fund kid it seems like you'd be able to bankrupt her pretty easily by just having a bunch of people spam the auction site. Or better, just start tacking on your 4 digit birth year to the price to up her cost per soul to a bit under half a million dollars.
And if she doesn't pay, well, I dunno how strong the consumer protection laws are in Japan, but it seems to me like the sellers should have some recourse to get their souls back if they can afford an attorney. Hell, since there seems to be a significant number of potential litigants and the cases have quite a bit of public awareness it's entirely possible they'd be able to find pro bono legal representation.
Anyway, don't take this sitting down, protagonist girl. You are not entirely without recourse.