Topsoil may contain fungus that causes damping off. Seed starting mixes are usually soilless.
Jim

Ashli, Jim makes a very good point. If you start your seeds inside, you may have trouble with the fungus. You can tackle that two ways. You can put the soil in a bucket and pour boiling water over it to kill fungus and weed seeds, or you can arm yourself with a mild brew of camomile tea to spray the surface of the seeds. The Camomile tea really works. Running a small fan can help too. But the top soil just isn't the same for starting seeds as is the seed soil.
If you winter sow, starting the seeds outdoors in the top soil should be OK.

GGG

Thanks Guys.
I started buying flower seeds a couple of years ago, just to experiment, to watch them grow...I started with Sunflowers and used potting soil and with some just cheap top soil...
Some of the flowers looked "spindley" as I call it,little growth...and when I took them from the plastic pot I started the seed in...they had white sticky stuff all over the roots...Guess that was the fungus Jim speaks of?...
I always use top soil to set out my annuals because the ground has so many rocks.
Think I'll try the seed-starting mixes and do it right.