If you use my stock all you really need to do is credit me, and let me know when you are done so I can look at it (preferably by putting a link in a comment on the stock used ).
Feel free to use my stock on sites other than DA so long as you change it in some way, and credit me.
If you want to make prints of your final creation and you've used my stock, go for it. You officially have my permission.
Just remember, some pictures may have additional rules, so make sure you check the artist picture comments on any stock you choose! Especially if models were used.