
Yo! So, actually~ real money WAS used last time we did this. People's real money amount was their cap, but no money was exchanged. When someone won a bid, they just opened a trade window and put up the amount they bid to prove they had it. Was pretty simple, but that's just one idea. Here's the old thread if you'd like some inspiration:
http://ffxiv-roleplayers.com/showthread.php?tid=940
http://ffxiv-roleplayers.com/showthread.php?tid=940