For selling, even via third party, you will need a cosmetic safety assessment. I use Scott Grainger (google cosmetic safety assessments and his name will be up top!) In order to have the necessary insurance to sell products you would need the safety assessment too.
Be wary of online recipes, double check with the soap calc as Julie says. When I first contacted Scott I told him that one of my recipes was in a book so "must be safe, right?" and he said "no!!! just because its on a book, online whatever by NO means ensures its safe at all..."
For selling, I have - 1) cosmetic safety assessments (its cost in total about £600 but thats soap, lip balms, bath bombs, baby soap...etc ) 2) insurance (about £71 per year) 3) membership of the soap guild 4) registered on the EU portal CPNP 5) Regular contact with my local trading standards (eg. soaps that look like cakes can be seized by ts...you need scales that are recognized by ts and in some cases callibrated by them)
I know it sounds like a huge faff but tbh it didn't take long to get it all set up and its a lovely feeling knowing my products are 100% safe and legal. I do get the hump when i hear of people trying to sell at car boots etc and who are flouting all the laws that are there to protect the public
However the Guild are bringing out more ways to shop such crooks and pave the way for those of us with a conscience