Lets take, for example, a niche for camping.
First of all we need to find exact trending product in US. We can make it by seraching ourselfs or we can make a simple search request in special service and search for keyword "camping", sort by US.
1. What we found?
2. Let's look at sales statistics
3. We see the dynamics of sales
Now lets go to Aliexpress and eBay, and analyze found product
The seller on Aliexpress sends the goods by the delivery service ePacket with the tracking number. The total price of the product will be $14.30 including shipping.
On eBay we see that price on exact same product is $39 and he got about 7000+ positive feedback, if we will look at his sales stats, we'll see that he got about 20 sales per day and got ($39 - $14.30) * 20 sales/day = $494 profit every day from only one product just from dropsipping.
Another example for not very popular niche i'm showing on video: