Oh! I'll explain it to you then. :0 It's the fastest way (that I know of at least!) to trade IGB. Basically the person (person A) buying the IGB will set up items (any items) at re-tail for the max price, which is 999,999 bells. The person (person B) giving the IGB will then buy the items and drop them outside for person A to re-list them. Cycle and repeat until the amount is reached. The max you can carry in you is 1,683,999 bells, so you'll only be able to buy one item at a time (usually the people I trade with just buy the same item over and over).
I have an ABD in a house right next to re-tail so that will make things even easier.
Also just note that 999,999 x 10= 9,999,990 so let me know if you want to me to re-list the item once more for 10 bells, or if you're fine with just dropping 100 bells.
I hope that all makes sense? Let me know if you're still confused about anything. And 25 tbt sounds great!