Hi, and welcome to ECF. Looks like Brother Bob got you started.
1) Battery safety is a top priority. There should be some instruction on it. Here's a good blog to get you started...
Mooch's blog | E-Cigarette Forum
2) Should inquire how much a new vaper has smoked (if they have) and whether they'd be likely to prefer a MTL setup or a DL setup. If they are unsure, I think an inexpensive box mod with a sub0hm tank and a MTL tank is the most inexpensive way to find their preference. They can always use one as a backup if they prefer one style or another. That is just my opinion, though....
3) Different types of e-liquids including Nicotine salts and WTA as well as explaining the need for lower nic on a high powered setup, and higher nic with a lower cost setup.
4) There is a pinned post here but basically also, what to expect when starting vaping and sx of nicotine overload as well as the need to hydrate, and what to expect during detox.
5) If you sell mech mods, full primer (and written information) on ohm's law , battery rewraps, and other safety issues.
6) If buying more complicated equipment, maybe a list of trustworthy review/demo youtube videos, so they can get help quickly if running into problems.
7) If they are buying a TC mod, instructions on TC including types of wire that are safe to use...
This list is getting long! But those are the things I can think of, off the top of my head...
Anna