Trading in stuff where you can source the product cheaply & wholesale, and sell it easily at retail. The normal trading mark up is 4 to 1 minimum, to whatever the traffic will bear—minimum 7 to 1 with jewelry. Maybe 1000 to 1 on software or high tech military products.
3% to 5% a month would be much too low a figure to aim for. I would buy big quantities of something, anything, at $1 and sell it retail for $7. If I did that every day for a year—well— you figure the monthly or annual return on 700% a day compounded!
Do you think Bill Gates made it by investing at 5% a month? Obviously, in passive investing where you do nothing but give or lend money to someone else, you won’t even beat inflation and thus, you will have a negative return.
Why bother? Just get a few precious metal coins or bars, and hide them.