Can I sell homemade natural insecticides, fertilizers?

If you're in the US, it depends on how you plan to sell them.

You can sell the "home remedies" (for lack of a better term), but you can't make any claims as to their nutrient content (for fertilizers), or their efficacy (for pesticides), without registering your products, and having proof of your claims.

Here's a link to the federal pesticide regulations:

https://www.epa.gov/pesticide-registration

Fertilizers I think are mainly regulated by state - visit your state department of agriculture for details.

So, for example, lets say you are selling cottonseed meal - you could sell it and label it as such, but if you want to claim it as a fertilizer, you'd have to have it tested and registered in order to make that claim. 

we have no idea what country, state, county (or equivalents) you are in, or where you want to sell the stuff,  so you have to check with your local authorities.