There are two proven ways to do this. One is to pick a very frothy market. For instance, about 10 years ago I acquired a preorder for four Butterfly Labs Bitco

There are two proven ways to do this. One is to pick a very frothy market. For instance, about 10 years ago I acquired a preorder for four Butterfly Labs Bitcoin miners shortly before they were to ship (months behind schedule but with the queue starting to ship). About a month later, received them. Sold them. Made a nice tidy profit. This was definitely speculation, but market timing worked in my favor. It could as easily have not. Second is one of my side projects I call "grains of rice". Everyone's pretty familiar with the cost of a sack of rice, but consider if your customer wanted just one grain. One grain total. Not interested in extras, no room for 'em. Just want the one. That grain of rice suddenly gets pretty expensive, as in hundreds, thousands, even tens of thousands, of dollars per pound shipped. There's additional packaging involved, and the frictional cost of shipping, and additional handling and listing costs involved, but it's pretty easy to buy things for $5 a dozen and sell them for $0.99 a pop. You can get an additional leg up by warehousing and shipping closer and faster than your source, meaning your customers paying your premium only wait the 3 days instead of the 3 weeks it took you to get it. That's not pure arbitrage, as you are adding value by wholesaling, splitting things up, and repackaging, but it's not uncommon to have eBay on one or both ends of a successful corresponding transaction.