[Westheim]

In the end, people WILL exchange cards this way anyway, but what do you want to watch out for? Normally you'd say why shouldn't they; the problem comes from abusing the system, when two accounts are really just one guy shuffling Gold+ cards to his primary account.

You probably want to watch for auctions that end in quick-buys within seconds, either for very low or very high PP and involving one account again and again. Filter out all quick-buys that end in under X minutes for Gold/Silver+ cards, then have the system check the user names.

I would never do such thing (and I have spoken out loudly against even single-player cheating big and small in the past), but I have a PC and two laptops at home. Steam accounts are free, and in a sale two more copies of OOTP19 would come only 20 bucks, and I could even have all three accounts running at the same time while feeling very important to myself... with such a setup you can snap cards back and forth in seconds at even random prices because the odds that someone catches the auction by accident are near-zip.

Do the quick-buy filter thing above rather than time-limiting quick-buy auctions. The only other efficient way to disable this cheatery would be to disabling quick-buy auction altogether, which would be a terrible solution.