suggestion
if zeny coin is a No no..
1. If we cant max the dealing for zeny.
2. Why not make a special NPC to conduct such trading? Its like you deposit the item to NPC and the other party will withdraw it using zeny. Then you can withdraw the zeny after.
3. Voucher System. Its like a ticket that allows you to make 100m to 1 ticket that if you logged out will be converted back to zeny. And has 10mins life, if expires will be converted back to zeny via rodex or automatically.(can lead to a lot of bugs, but can be improved)
4. Deal facility room. That can be hired if with party and cant be exit with, by logging out. You can both exit only when both party press a device and approved by both ends..