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They are small and brown to dark brown in color—found in unopened pine cones. If the cone has already opened, chances are the seeds have dropped out.

Sounds like a spittle bug problem. Here is an article that you may find helpful: https://www.gardeningknowhow.com/plant-problems/pests/insects/spittlebug-control.htm

It would be possible to grow a tree from seeds that you bought.

When and how to start the seeds though would depend on which variety you are growing. If you can let us know that, we can get you directions on how to do it.

This article should help: https://www.gardeningknowhow.com/ornamental/trees/tgen/how-to-remove-tree-sap.htm

Do you know the varieties and, other than brown needles, are there any other signs of problems, such as bugs or anything like that? This information will help us diagnose the trees.

It sounds like the trees may be succumbing to a blight that affects that particular kind of pine tree. I could be wrong because this blight normally only affects part of the tree, but any bight, if it is severe, can kill a plant. This article has more information on it:
http://www.unce.unr.edu/publications/files/ho/other/fs9440.pdf

Pine trees tend to have acidic soil, but most plants can handle the acidity. The real problem will be the fact that the tree will suck up all the water. You will need to make sure the plants under the tree get extra water to offset this.