I feel like they should know what the differences between high and low end cameras are. As well as the differences between lenses.
For example, the Canon side has the Rebel line for the entry level. The Rebel XS, T1i, T2i, T3, and T3i. Then they have a mid range tier, 50D, 60D, etc. They should also know that all cameras I've listed so far are crop frame cameras and work with both EF, and EF-S lenses. Moving upward, they have the xD line which is more geared towards professionals. The 5D Mark II, 5D Mark III, 1D series, etc. Those will ONLY take EF mount lenses because they are either full frame or 1.3x crop factor. However, as a curve ball, there is also the 7D which is a crop frame camera geared towards sports shooters and CAN use EF mount lenses AS WELL as EF-S.
They should also be familiar with the same set of information for Nikon cameras, which I am not going to go into great detail with. But they have a similar tier system, but with a few quirks that are different than Canon's. You can use any Nikon lens on any Nikon camera (AFAIK) but some are DX lenses which will only utilize the central area of the sensor on pro level full frame bodies.
They should know what a fixed aperture lens is. Like a 70-200mm f/2.8 (fixed aperture) vs a 70-300mm f/4 - 5.6 (variable aperture). They should also be able to respond to customers as to why the fixed aperture lenses are far more expensive than the variable aperture lenses. Which is directly related to the cost of production. It is much harder, and requires much more engineering to make a lens that has a large maximum aperture throughout the whole focal range, and the quality of glass is generally extremely high.
Knowing their way around cameras of all the major manufacturers would be helpful, but that's something they'd really have to learn in-house because most people only have one camera system. I have $12,000 in Canon gear, but I can work my way around Nikon's, Pentax, etc. The rules of exposure don't change, just the menu system and features.
They should also know about flash units (Speedlights, monolights, etc). What TTL metering is. What METERING in general is. As well as flash accessories like umbrellas, softboxes, beauty dishes, and so on.
If I were hiring an employee for this position, I'd also like them to have a good idea of what the exposure triangle is to help customers troubleshoot camera issues. Firm, working photographic knowledge, even with lighting would be nice to have in an employee.
What's this job paying? I'd fly out to Vegas to sell cameras assuming it's a salary I could live on.
