Okay, you need:
1.7M on the account that is receiving the money & Karamja gloves 1
You have to go to the Grand Exchange and buy Obsidian Maces, Obsidian Swords, Obsidian Mauls and Obsidian Staffs.
Go to the Tzhaar place and sell them to the shops for tokkuls, you will need 260,000 Tokkul.
When you have 260,000 Tokkul, buy an Onyx from the Tzhaar shop.
Sell the Onyx to the account that is giving money for 2.5M
This means you spent 1.7M in order to buy the items, but received 2.5M
(The account with the onyx would have to get Crafting assist request and craft the Onyx into a Fury which is worth about the same as you spent to buy the Onyx)
This means you transfered about 800k right there
You're able to do this 10+ times an hour
That means 8M+ an hour
(Note: Do not try this method with under 1.7M on the receiver and 2.5M on the giver, you will not be able to transfer.)