A repeatable product sale is the idea that you can post the product description, price, and photo once and then sell many copies of the item. This approach takes less time to manage your website. For example: a tshirt with a custom design, just call up the screen printer for more tshirts. Selling prints of artwork is repeatable. Selling the original one-of-a kind piece of art is unique.
It is still completely viable to sell unique items. It just means that your business model is going to require more time to photograph, post, market, advertise, mark as sold, and then replace the product with the next one. Just build the time requirement into the business. An example of unique items: antiques, collectibles, original art work, original hand-made crafts.
Start thinking about what products you could sell online. So far, we have mentioned that it should be somewhat unique - something a person cannot find at Target, or the grocery store, or the hardware store. Competing on a commodity product means competing on price against huge stores that can sell it for less than you can buy it for. But, products that are repeatable, or even hand-made from a pattern, are easier to sell online that one-of-a-kind items. So, getting the right product for online sales is a balancing act.