If I was selling a bag of flour and sugar to a CI who thought it was meth or coke can I get in trouble? How or why when I am selling a legal substance?
Yes. Selling something advertised as a prohibited product, whether or not it's true, is technically a crime. Assuming we're talking about the US, in most jurisdictions this would be treated just about the same as if you were actually selling the same volume of the advertised product. I believe the reasoning behind it is that even if it's fake, you're creating a market for the real product to be traded, or something like that.
So if you sell a 1 lb bag of flour calling it cocaine, you'd get charged as if you sold 1 lb of coke. The final judgement may not be as severe as actually selling a pound of coke, but the initial charges typically will be.
Also, flour and sugar do not look like coke or meth up close. It would be immediately obvious a user/seller the moment it's in their hands, so this is also a really good way to get your ass kicked.
I've not handled meth, myself, but I know that coke usually clumps up into flakes that have a "fishscale" like appearance to them with the way light reflects off of it. That slightly pearlescent appearance is the first thing a buyer would be looking for, because there's not a whole lot of other white powdery materials that look like that. It also clumps up with a consistency slightly similar to something like Comet cleaner, so it wouldn't be as fine-grain as sugar or flour would be.
I dunno why I'm helping you disguise your fake drugs lmao, just be careful.
Unlike Chozo, I have a firsthand account of meth (interned at a crime lab in Idaho for my college degree). The meth we received was in crystal form. The really good meth was much more clear than the impure meth. Impure meth was cloudy, and without having had access to both kinds this next point won't mean much, but upon pouring the samples out into the table to take a smaller sample, the more pure samples sounded nicer. The impure stuff sounded dull. Now, finely grinding the meth into a fine powder MIGHT help disguise it in a sample of other white substances, but that came with its own issues. When I was there, if a sample of a whole was found to contain a controlled substance, the entire specimen was considered a controlled substance (something like the excuse that pot was being shipped with tomato plants was storied to me). I know I got a little off track, but the point is that meth has vastly different properties to that of sugar or flour.