Hi FTB, welcome!
I don't really know for sure, but if you are going to harvest those roots, August seems like a good time for me.
I am not familiar with the PA laws. What is posted in the LAWS section was done last year and I can't guarantee that it is current at this point.
I don't think you will find goldenseal seeds for sale, and if you do, don't buy them. The germination rate for seal is very low (I seem to remember in the 30s%), and they have to be sowed right away while green. You are much better to buy them as roots for transplanting. You can often break them up again in a year or two and expand your planting again.
I will normally open for orders of seed and rootlets about the first of August. However, with the crunch in seeds this year, I might open in a few weeks for larger orders (with an early discount of course). I don't know what will be available for digging until the middle or late July.