I didn't try selling mine until they where a good 4 inches in length . They all had their legs etc and where eating a good varied diet of food...i.e chopped worm as their main diet plus very occasional pellets, lean meats and shrimp. I think they where approx 4 months old, and I supect the bigger they are the better they sell.
If you are struggling with your first 40 I would not try hatching any more as looking after 40 can be a bit time consuming, also its best to know that you can actually re-home the first. I was fortunate in that I had an aquarium that could easily take on and sell mine.