Hello. So, I've built another house and I decided to put some OFB and Seasons objects, but also some objects from the base game that are not normally buyable (such as mugs or pens). In order to make them buyable, I had to get some type of mod (or how to call it) on MTS. But when I tried placing them in my download, MTS upload section recognises it as custom content. This is one thing. Another one is that I don't even know how I'd label this since these objects actually aren't even cc since they're just from the game by default, just made buyable. What should I do?

You have a mod that recategorizers items to be buyable. You should be placing those in program files not your downloads folder. This way you cannot corrupt your game by trying to delete in game and they won't try to package with the lot.

Sorry, did not use the forums for some time, but thank you a lot! I hope I have done it properly.

You should probably avoid using those objects in houses you intend to upload. People are most likely going to need the same mods as you to make the items appear (if the items are important for the overall look of the lot, make sure to link any needed CC/mods in the upload, so people can decide whether they want them or not),. If people don't have the same mods or EPs as you, the objects could end up being replaced with some other object ingame, which can look rather messy. It depends on the objects.