I have been trying to find information on this subject including contacting designers. I have heard as long as you don’t include the instructions in your sale, all is fine as that is the intellectual property. But I have also seen instructions barring against selling of the physical model as well. Most information I find is on the piracy of the instructions themselves. I am looking for info on the selling of the actual build. Like, what if I were to purchase instructions of a modular building and after I buy all the parts and build it I see that I don’t like the look of it in my city. Could I not sell the model to someone else? I believe to understand the intellectual property laws but what about fair use? Thanks for any help.
I'm not a lawyer, so this is purely from a layman's understanding.
You don't say what jurisdiction you're in, so I don't know if what I'm saying applies to you. But in the USA, the First Sale Doctrine means that you have the right to sell your copy of any copyrighted material that you have bought. If, for example, you had bought a physical instruction booklet, you would (in the USA) be within your rights to sell that booklet to a third party, with or without the bricks needed to build the model.
If you received the instructions as an electronic copy, you would still be within your rights to do this, but would need to ensure (and ideally find a way to prove) that you had transferred all copies of the instructions and not kept one back for yourself.
I don't know if there is any precedent for the scenario where someone sells the model (built) and keeps the instructions. Intuitively, I suspect this might violate copyright, as the completed model could be considered a derived work of the copyrighted instructions; either could be used to make more identical models.