It stands to reason, that if there was one magic thing that people buy, then everyone would be making it.
But, if you want to focus on how to make what you make more marketable, it's a matter of ensuring orignality and marketing it to the right people.
It's one thing, for example, to make earrings. People who make and sell jewelry online are a dime a dozen, and it's easy to get lost in the shuffle. Look at what others make - what makes the earrings that stand out to you, stand out?
When you design your own pair of earrings, keep in mind: Is your design based on the most inexpensive materials you can get? Are you throwing together a pair of earrings haphazardly? Are you making that pair of earrings simply to destash leftover beads? Are the materials you're using common, and are similarily used in dozens of other pairs of earrings available where you're selling? If the answer is 'yes' to two or more of these questions, there's a good probability that the earrings you're making aren't going to stand out, and are likely to sit in your shop for a long time, regardless of how or where you promote, and regardless of what sales you may run in your shop.
I can't vouch for what everyone buys, or even what the majority buy. I can say, though, that when I do buy handmade, what I tend to look for is something that doesn't necessarily look handmade, but rather something that looks unique and original. Or rather - if this makes sense - if I do get into the 'handmade' look, it has to be something that looks handcrafted rather than homemade. In my mind, handcrafted means original and of quality, homemade looks like something thrown together haphazardly from materials out of the dollar store. It's not to say that what I look for is necessarily expensive or pricey, but I avoid buying anything that looks cheap.
I know that's not really a black-and-white answer, but for what it's worth...
