Hi, OK, many thanks. I wonder whether day old chicks may be better as with some breeds you can tell cockerels apart very early on? (so I've read!).
Do you mean buying as day old chicks, or selling as them?
If you buy day old chicks from a laying breed (as opposed to meat), they will usually have been sexed at hatching, and the males removed (let's not beat about the bush - killed) already, so you will only get females, which you can then raise to point-of-lay and sell.
If you're hatching them yourself, you have a choice of hatching a breed or more likely cross-breed where you can tell the sex on hatching and get rid of the males yourself (known as 'auto-sexing'), OR a breed (most likely a pure breed) where you can't tell when young, and need to wait until they're a few months old before you can accurately sex them.
If you're still starting out and learning, I'd start by buying some good parent stock of a hardy breed that you like, hatching a few, advertising locally and seeing how they sell. You won't make much money that way, but it will get you started. As long as you don't go over fifty birds, there aren't any regulations to worry about, but do make sure you have a plan for what to do with the boys first. I keep seeing facebook posts along the lines of "five young cockerels for sale - must go to good home, only £10 each" - don't be that person!!