You can buy your stock from a wholesaler and list them on your online Selz store for customers to purchase. You will need to describe the features of each product in detail, and, since headphones are something of a fashion statement, include photos. Because this product is in high demand, with the right prices and marketing, it should sell fairly easily.
Using your skills for profit is a common trend with all of the best side business ideas. If you're an expert at something, there's likely an audience of people online who would be willing to pay to become an expert in your field—just like you. If you want to take your skills and turn them into an online course that teaches others how to get the same results you've achieved in your life, career, or business, start with How to Create an Awesome Online Course on Udemy, where instructor Miguel Hernandez covers how he makes over $90,000/yr teaching online. You'll learn from more than 8 hours of video instruction.
I created an “Every Day I’m Hustling” shirt and sold enough for the campaign to ship, but didn’t earn much on the experiment because of some not-very-effective Facebook advertising. Still, some opportunity here and a friend of mine is doing REALLY well with this. If you can create awesome designs and reach the right people with good Facebook targeting, Teespring can be very profitable.