My general understanding on this is that the model/design must be a smaller part of a bigger production that one made him/herself or with help of other content purchased, so that the produced results can be considered a new unique work.
So simply downloading a product/scene/design and take some renders of it and consider it a new product would not suffice.
(I would definitely not approve if somebody did this with my work)
I think its safe to say that the general rule of thumb is that there must be some bigger scenery, composition, game or project where the stock model would be a smaller part of it (say +/- comprise 30% of it at max).
So a product on its own cannot be used to simply derive a new product from it without sufficient additions or alterations so that it could be considered a new unique work (say +/- add 70% new stuff or alterations in the project).
For example, I ones had a discussion on this with someone offering kitbash sets and he told me that only 30% of his set could be used for a model that would get in stock media and only on condition that the model itself would only comprise of maximum 30% of it, so 70% of the model would need to be parts made by oneself or from some other kitbash sets.
So in general in this case it would mean buy 3 to 4 different kitbash sets and only use 30% of each of them to make new models.
However, keep in mind that there would be artists that would not allow a single part get in a stock media model, so its always wise to ask/negotiate with every artists and get written permission/license before using their work to derive new products for stock media.
Remember, in cases of dispute only a judge can decide who has it right, but you want to avoid coming it to that right?
