1: Though I'm sure you already know this, there's no need to ask permission to use my bases. As long as you credit me in the description of your work, that's all you need to do.
2: Speaking of crediting, let me explain how. In the description of your work, just put something along the lines of "base by @ Priest-bases" (without the space between "@" and my username). You can link back to the base if you want, but it's not necessary. However, ONLY linking back is not enough. I can't see your work unless you mention me.
3: Using my bases for art trades/commissions is fine, but only if the person you're giving said piece to is alright with you