A reproductive swarm is programmed to build comb, get the brood nest established and get ready to reproduce again. So, it will pay big dividends to feed a swarm and let them build comb as well as build up their population, (but stop feeding before they begin backfilling the broodnest).
In my hobby experience, most NUCs are put together just before sale. The recipe for making a NUC for sale varies a lot from region to region and person to person. But, you need a laying queen, brood of all ages, and some stored honey and bee bread for best results. Weather and honey flow will dictate how much food reserves they will need.
In the spring of the year when nectar and pollen are easily available, hives (read NUCs) can grow rapidly. Less so later in the year, but splits can still be made and swarms hived with good success, but they may need to be fed if there is no honey flow. Many beekeepers remove resources from NUCs to keep them from swarming due to lack of space during periods of plenty.
In general, my NUCs take a bit more care because you are trying to run a three ring circus in the space of a camping tent. HTH
P.S. FWIW, I ask the buyer to bring his equipment over and I put the NUC in it and make sure the NUC is established in his equipment (usually new equipment) before he takes it home. I sell so few that it works well that way.