Buy trade-locked item.
Once the item is tradable cancel item reservation. You will be credited 101% of current item price to your balance.*
Repeat step 1 & 2...
This way if you manage to do it every day you can get total return of approximately 134% balance invested just after 30 days!
*Important: Please note that it is 101% of current item price when cancelling reservation. It means that you can receive more or less than 101% item price when reservation took place - prices keep changing every day.