EULA always comes first! You are not obliged to give credits to any of your renders for an item you have purchased.

However.. if it is a commercial resource that you will use for creating another product of your own, then yes, you do need to give credit to the original artist.

For example, a skin resource let's say by 3Dream.. you do need to keep his logo on the texture maps you will use as base, even if you heavily modify them. If the vendor prohibits the use of it for freebies, then also yes, you need to abide by his wishes otherwise you're violating his copyright!

I hope things are a bit more clear now! :)

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