Well, people regularly make trades in the 500m~1b+ range all the time via trades set up in Eden/forums... I was extremely nervous with my first 1b+ trade (obtaining Deviling carded garment), as I simply had to trust the other person and hope that they actually stuck around for the 2nd trade with the item I wanted. Intense, lol! >.>
Many years ago, when it was still Gravity, I remember when they would shut down a zeny selling site, Gravity would subpoena their sales records and use that list to ban all of the RO accounts that had transactions with them. Oh, and the account would show up on the good ol' Ban List, hahahah. :]
If you want, we can trade back a billion zeny a few dozen times to see if a GM shows up. I've tried it before, and it didn't work LOL. xD