That's the #1 factor. Availability. There are precursors that are worth more than gold because I am not set up to do a nitrogen atmosphere or 10 atm. If pressure. Those same chemicals are not as valuable to someone who is.
If you know nothing about chemistry, those pseudoephedrine pills are valuable, but if you know how to make P2P, they aren't valuable at all. I've considered the option of making reagents and precursors instead of product, but it's a lot more niche market and it's right In line with making finished products in a profit/ danger scale.
If I had absolutely nothing else going on at home, I'd probably farm yeast for L-pac in a huge scale, or farm poppies, ephedra, khat, or stuff along those lines