Set a maximum amount of gil that can be carried over to a non-legacy server and bind all equipment to the player who transfers.
For example, assume that the maximum amount of gil to be transferred to a non-legacy server is 100K. A player who has 1 million gil on a legacy server would need to create a mule to hang onto 900k or sacrifice it all during the transfer. The 900k left with the mule could be utilized by a new character created on a legacy world if the player so desires. Once the player has reached the new world, all items and equipment are forever bound to the player (can't sell to NPCS, can't sell to the market). Therefore, it's in the players best interest to liquidate while on the legacy server if that player has items that he/she wish to sell (gil from these items would need to be left on legacy server assuming player is at the 100k cap).