Fetes and craft fairs still want you to have a product and public liability insurance for you to sell them in the uk. I phoned the company where I purchased the soap base they said they are insured but not after you add essential oils etc in case you add to much and that could irritate the skin.
You most certainly can sell melt & pour. Many do and do well with it. You will need to make sure you have insurance and tax info in place as well as any local/state/country certification's if needed.
I done a stall yesterday at a fete! It was a very hot day the bags with my soap in all steamed up and the sun took the colour out of the soap! Very disappointed! X
It's best to use items that are safe for the skin--those marked for cosmetic use only, non-toxic, etc. That's why I highly recommend micas, even though there are many other ways to color soap. Always read what the colorant contains and a reputable supplier should list this on their website or be able to email you the list of ingredients.
I would not use crayons in your soap. Especially if you are selling. You don't know what's in the crayon. I too would only use colorants that are skin safe and recommended for what you are making. Micas, Oxides, Ultramarines or M&P colorants from reliable sources.