"Drop Shipping. You make a website that specializes in something (for me, it was camping supplies and slot machines) and list all those items for sale. What you're doing is basically acting as a middle man between the wholesaler and consumer (essentially retail but not really since you don't purchase in bulk). When they buy the item from you, all you do is go on the wholesaler site that you get your [things] from and order that same product using the money they gave you to pay for it. Since they paid a marked-up price, you just keep the difference.
. . . I did this during high school and college to make extra cash. Whenever I sold a slot machine, I would usually pocket like 50 bucks. I remember one day I sold five slot machines and, like, six camping tents. I made like $500 in one day and only had to enter the shipping info for 11 customers. You can usually bank $1,000 to $4,000 per month doing that small scale. The big guys who do this full-time usually have 10 to 50 sites."