If you run a shop you need a pet shop licence but a hobbyist selling creatures that have outbred your tank don't need a licence. Snails, fish etc can be sold to shops, or on here in the Classified section or even on eBay. Shrimps and snails can be sent by Royal Mail unlike fish which have to be sent by specialist courier.
It's more lucrative to sell shrimp and very easy, I want to mention that the 7 or so snails I saved over into a 150L AQ became very huge, with a perfect formed shell, after they had laid eggs and the population increased they became still smaller and reduced, they reached the pest dimension soon, in passing of time it will change and the maximum in my case is over, today they have a stronger rivalry of Melanoides (which are really usefull snails) and recently shrimps that will help reduce these snails enormously. Because of predator push they have developped a strategy of reproduction that make them, similar to the shrimps, to aquarium-plage-animals.