How can I store PVC pipe in the sun and prevent UV degradation?
PVC breaks down when exposed to sunlight as do all plastics.  Some PVC-based products have additives to slow down the degradation but there is ultimately no way of stopping it.  Left exposed long enough, it will break down.
Your question, I think, is really: &quot;How long can I leave PVC exposed to sunlight before the degradation affects its marketability?&quot;
The answer is: &quot;It depends!&quot;
It depends on the individual product and its material properties as well as things like the intensity and duration of sunlight.  (Think Seattle, WA vs. Tucson, AZ)  It's also going to depend on the opinion of your customers and how much exposure they will accept.
I can only see two options:

Reduce the amount of time the PVC sits in your yard.
Cover the PVC somehow so that it doesn't get exposed.

There are no &quot;miracle&quot; solutions that I am aware of.