Net material if sold as material only is almost guaranteed to be safe to use. Just as people who purchase fabrics can use them any way they like and sell finished clothing, other fabrics may be used in the same way. Net material is almost guaranteed to be used by multiple manufacturers of fishing nets...and multiple manufacturers of products totally unrelated to fishing.
Metal parts, again, if sold in bulk as components, and not as replacement parts for existing products, again, are generally safe. That would be in the same manner that nails, screws, and other types of hardware are sold and used as parts of other products sold.
For example, if you are able to purchase 100 or 1000 pieces of a particular component, then that is tacit permission to use that component how you see fit and sell products containing that component. If you cannot purchase the component in bulk that would be an indication that it could be proprietary and you would need permission. However, at the same time its unlikely that a competitor produces that component themselves, and therefore if you are able to find and purchase that component from the original manufacturer, again, you are home free.
Therefore, when you are looking for sources for components, you need to be looking for the actual manufacturers (or the distributors for those manufacturer's) rather than looking at your competition as a source.
If you can buy it in a hardware store, and it does not have a logo stamped on it, then you are also home free even if you only purchase one at a time.
The reality of things is that very few manufacturers produce all of their own components and those components that are proprietary are generally mechanical to one extent or another.