When I have bought them I intentionally get juveniles, perhaps 3/4". Haven't sold any, but I like the younger shrimp. They seem to adapt to a new tank faster and of course, you're getting the shrimp for nearly its entire life instead of whatever fraction of the adult's remains. I think as long as you state clearly the size of the shrimp you're selling you shouldn't have problems.
With a shrimp as prolific as the RCS I think that the life span of an individual shrimp is way over-blown in the argument on what age to purchase but the juveniles ship and adapt better so that takes precedence anyways.
I noticed you were from SC, I'm in the columbia area and brand new to site/hobby. I currently have a very low tech 10g with a few CRS and amanos and was thinking of starting a small RCS breeding colony. Let me know if you decide to sell some off as I would be happy to take them off your hands.
Also, I've been lurking the forum for quite some time but this is my first post so hello everyone!