It really depends on if you need the milk early on, and if the kid(s) are one the bottle. Once they are safely on the bottle you can sell them to someone who can bottle them with either their own goatsmilk or with milk replacer. Realistically most people sell once their kids are off milk, probably 12 weeks onwards, and they are well established on hay and some kind of concentrate.
Also if both parents are registered with the BGS, you should register the kid(s) if you are a BGS member, as that would increase their value.
If you have found a buyer, make sure they have a CPH number.
I find selling goat kids really, really difficult - hence first goat house extension last summer...