I think there are a number of factors to consider. The species makes a big difference in size, so a 2 month old veiled could very well be eating crickets that are the same size as or even bigger than what a 3-4 month old panther would eat. Plus the individual chameleon size can vary hugely in clutches. Then there is the experience of the new owner. If they have kept hatchlings before, then they can probably deal with a young chameleon (as long as they are okay with it being that young.) Then there is the whole ethics vs. price thing. It's cheaper to sell a chameleon younger, but morally it's not so hot (usually.)
I think it's totally inappropriate to sell a young chameleon without the new owner/buyer knowing exactly what they are getting into. Just because a chameleon has made it through the first two months of its life doesn't mean it will make it to month three. Once you get to 3 months old the chances of them living seem to be much better than when they are younger. 4 months is even better!