There’s an excellent chance that you have one or several musical instruments sitting in storage that haven’t been used in years. Perhaps it’s a leftover from when you were in school, or even from your days playing in a band. Whatever the reason that you have it, it’s probably worth money if you can sell it. Brand-new instruments are ridiculously expensive, so people often look to buy used equipment instead, particularly if they are novices.
If you already have a website or blog, look for vendors that offer related but noncompeting products and see if they have an affiliate program. Stick to familiar products and brands—they're easier to sell. For promotion, place simple text or graphics-based ads in appropriate places on your site; you can review or recommend products or create a dedicated sales page or website to promote a particular product.
If you’re looking for inspiration, my friend Michelle Schroeder-Gardner of the website Making Sense of Sense has become the expert on all things affiliate marketing. Michelle earns more than $100,000 per month from her blog and the bulk of her income comes from affiliate sales. Michelle has had so much success with affiliate marketing that she even has her own course called Making Sense of Affiliate Marketing.