I have a few sacks of a seed starting mix, it's very fine so only use it for tiny seeds like begonias, primula etc. the rest in what ever is handy, usually Sunshine mix #4 for beans and like, for something smaller I usually sift a bit of this and add a bit of sand.
Annette
Once, I used an off-brand from the hardware store. It very nearly wrecked my entire season! I had just enough time to start over once I realized that something was wrong. I probably lost a variety or 2 for that year, because I didn't have more seed! Now, I'm skeered to do the starting with anything other than Black Gold.
Soil for plants that will be in containers after they are about 3 months old can use homemade soil mix - compost will be the most important ingredient and highest percentage.
I use a mix of store bought and soil/leaf mold/compost from here on the land. I like the seedlings to be exposed to the local bacteria and fungi from the beginning so there is no shock or recovery time when the plant is exposed to a nonsterilized soil culture for the first time upon transplant.
I've never noticed my seedlings having problems with shock or recovery time. Perhaps that's because the soil in the raised beds is closer to SFGardening material then local soil
My brother only uses store bought...the most expensive he can get his hands on~around $50 a bag...and has this fancy grow light stand that cost him almost $1K, a soil block maker, etc. and he's always amazed at how quickly my tomatoes grow when set in the garden...they surpass his that he has grown three times the size of my seedlings before he actually plants them. His look great and are sturdier looking plants compared to my seedlings but I still have more rapid growth in the garden than he.
I grow mine in weak ol' window light on heating pads, most of them in porcelain wash pans and restaurant meat trays.