The best advice i can give is have corals that are beginner corals but not ugly looking something other than grey leather or finger coral's
i'm more of an sps guy so a store that doesn't carry sps is a store i will spend less money at, that is where i want them to fill my test kit, salt, dosing and fish /coral food, i don't mind going to a store more geared to softie's and lps/zoanthids if they carry the other supplies i love to use.
Rod's reef food and coral food are the best i've ever used for fish food and coral but on the expensive side, other than that any place that carry's DT's pyhtoplankton and oyster eggs, or the new stuff i'm using now which is the Reef Nutrition line of coral food, if i could buy a better salt than IO reef crystals locally i most likely would switch salt brands, carry B-ionic or BRS line of 3 part and make sure to sell ELO's test kit's as ELO's are the only test kit i would ever recommend for a new or older hobbyist they are acurate easy to use and easy to understand even for a beginer, maybe have a write up or booklet you can hand out to break down the most important things to make a customers reef a success.
Such as giving them parameters to keep in check like Alk,Calc and Mag (unless waterchanges are enough for what they have stocked)
the proper salinity and how the evaporation process works as salt doesn't evaporate
The importance of lighting and photoperiod, light intensity
The importance of having a ton of flow/circulation
The importance of doing at least a water change every week no matter the size of the tank
a break down of what fish are appropriate for what the given tank size in gallons is
getting that information upfront and from the store owner in a booklet would make it easier and simpler for new hobbyist that don't research on forums to understand how to be sucessful even if they aren't keeping corals that considered HARD
Either using good water or buying a RO/DI machine a lot of stores i have been too will sell RO/DI water as well as water already mixed with salt, but it would be nice if they carried at least 2-3 of 1 good 5 stage RO/DI to sell to hobbyist