Using paid models that where once free

I think that what makes you can use those models in your game is not that they are free, that is indicated by the type of license.

I mean that the model is free does not mean that the use of it is also free, so my advice is Check what type of license the models you have purchased.

It would also be convenient for you to contact the seller, so you will make sure that the use that you can make of these models is very clear.

Change of licensing does not apply retrospectively, i.e. you are obliged to the license that was actual at the day of purchase or free download of the model. It doesn't matter if the license was changed after purchase/download, or if the model was completely removed from the market. Probably the only exception to this rule if the model in question was sold illegally, e.g. the seller is not the author of the model and has no rights to sell the model, or if the model was violating some rules of the marketplace where it was selling. If that's the case you have the rights to seek compensation for your expenses and maybe even for the damage that may have caused to you by that situation. Having said that, it's important to mention that "free" is not always free, like TheGoodBrick already said. By default free models on cgtrader are distributed under Creative Commons license, which means that at very least you have to give attribution to the model's author if you want to use his model in your game. Unfortunately cgtrader does not specify which of the few available CC licenses are exactly used for the free models, that means that you still need to contact the seller and clarify things before even planing to use such models in the game. Some versions of CC license are quite restrictive in what's allowed and you want to make sure that you won't get in trouble after your game release. Also don't forget that if the gun models are not fictional, but created after some real-world guns, their design is most likely protected by third-party IP and you must get clearance from them on usage of such models first. Game publishing is serious business (even if it's small time game from one indie maker) and you might burn pretty badly if you'll do things that goes against the law.