If you buy the royalty free rights to use/edit a 3d model and textures in "any commercial software or application you develop", so long as the buyer doesn't have access to the model(must release as exe, apk, etc), and you can't resell the models on a competing website.
Now say that application is a sprite sheet generator that has 3d animated models that snaps a sequence of screenshots at different poses, lines them up, resizes and packs into a png file....hence a generator.
Technically it's a standalone application and the users don't have access to the models without breaking the law. It's not in direct competition because the modeler doesn't sell 2d art in any way.
How would you approach artists for models you've already bought? Would you restate to them that their terms didn't mention 2d art, then tell them about your app, or what? They don't even list the terms anymore on their site, I have a 3+ year old version but there's currently no terms listed when you buy models.