I do not think you have to go by a time so much. If the gecko is eating good, growing fast, is alert and healthy you could sell it sooner. say 10 grams at a reptile expo. I would however make sure it is at least 15 grams before shipping...
For me each one is different. If Im at a show and selling super hypos, then it is like Dave said. If I am selling a Raptor and tell the person what sex it is I want to know for sure what sex it is. Plus some stuff I just want to see the changes it goes through before selling so maybe wait till 20+ grams.Hope this all makes sense.
Yes, you definately want to see what the coloration will be if it is a higher end morph. that makes all the difference. if they are just normals no big deal...