I routinely grow bag seed that turns out great.
If you can get pictures or samples to go along with local seeds, i think they are just as valuable as the online guys.
There is more than just this or that that can make a great plant.
Seed selection, Light, water, nutrient, root zone, trimming, timing, no disease, no pests, ect. all make for great end product.
If your plants are at least 3 foot tall and or wide by July you will have a substantially larger harvest than something that was started late.
Little things here and there won't doom a grow, but the less that goes wrong and the more preparation the better.
I would say the most important aspect of a grow is how experienced/intelligent the gardener is.