Two things about the seeds I buy. First, I like to buy organic microgreen seeds. This ensures that I'm getting a variety which is already a favorite microgreen and is free from pesticides. I also look for seeds that have been tested for pathogens like e. coli and salmonella.
Buying a microgreens-specific seed also can give you more information about the variety, like taste and time to harvest when growing as a microgreen. This can be very important so pay attention to things like germination time, whether you should pre-soak your seeds, and how many seeds to expect per ounce.
Not every vegetable seed that you would plant in a garden works as microgreens. The leaves of some plants are toxic, so look to the microgreen lists at places like Johnny's Selected Seeds and True Leaf Market as a starting point for what seeds make great microgreens.
When buying your seed, look at the time to harvest. Microgreens take different amounts of time to reach the harvest stage we talked about earlier. Shorter time to harvest means it will be easier to grow. Certain microgreens can be harvested in as little as two weeks, which leaves less time for error. I find myself constantly referring to this chart from Johnny's Selected Seeds because it shows me time-to-harvest and flavor.
I love the smaller seeds. You get lots more per ounce, making it easier to experiment. Greens like arugula, kale, broccoli, collards, and mustard are small and easy to grow. Sunflower, pea shoots, basil, and cilantro are also popular microgreens that can add lots of flavor to a dish.
For more extensive list of the good choices for microgreens, I suggest browsing some of the sites below below because they have sections of their site specifically aimed at microgreens growers.