I recently read through many posts on this forum, and I discovered that many people (spunky1, others) use pine bark as the main component of their potting medium (or to fill their seedling beds).

I'm in California, and I don't think that I've ever seen pine bark for sale here - as mulch or as a potting amendment.

We do, however, have redwood bark (and/or pulverized redwood) available in abundance. It is sold here by the bag as mulch, though the smallest pieces are not as small as the "dime sized" pieces that were mentioned for the pine bark. (The smallest "mini-mulch" that I have seen here is about 1/2".)

Has anyone out west here tried redwood bark (or pulverized redwood) in an equivalent potting mix (which also has sand, lime, time-release fertilizer, processed chicken litter (not sure what our local equivalent to that would be), and alfalfa pellets (haven't a clue where to get those))? When using any kind of bark, can potting soil be substituted for the sand?

I can't answer about the bark, however, I will give you my opinion on the sand. The reason we use sand is so the bark doesn't stick together and prevent draining or prevent the water from going into beds/pots. Have you ever watered mulch and the water just rolls off? Same idea here. I don't think the potting soil would do the same thing as sand since the soil can stick together itself unless it has other things added such as perlite/vermiculite (sp?). Before I started using sand I bought a huge bag of perlite from the nursery/seed and feed place for like $20; It's called a 50lb bag even though it does not weight 50lbs.