Yes, as long  as you aren't selling the alpha texture planes themselves.

Yes. Our alpha texture stamps work in both Cycles and Eevee. Cycles offers much more realistic results.

Yes, all of LightArchitect's products on Blendermarket include free updates when you purchase through your Blendermarket account.

No. These texture plane assets are extracted from grunge in real world photos.  Several were generated and painted in photoshop.

The texture resolution varies. Some are 4k and some are only 1080p. Most are at least high resolution and will work for detailing assets in the mid and background of your scene.