What Do Shops Not Carry?

you can never find redstone repeters

 

I tried selling those for a brief moment in station46 (during Cyber Monday and for a few days later): but I sold none, despite massive advertisement.
In general, I think that most items and tools that can be easily crafted with raw materials do not have a huge market, they might even sell for less than their worth in material: crafting is not reversible, so you can craft by yourself using (bought) raw materials, but you can't get raw materials from crafter items.
The only exception to this rule of thumb are items involving non-stackable ingredients, which are a pain to craft in massive amounts: for instance, I sell quite some dispensers, although inconstantly, for a much higher price than their worth in materials.

 

I'd suggest selling something painstaking to make. Like, @Expipiplusone said, try dispensers (although with 1.12 crafting, it's now much easier) or, perhaps, concrete, since you need to water it. I remember noticing that some stations sell color blocks for more than they're worth in the blockshop at some point.
Also try something that doesn't usually lay about. I, for one, never have daylight sensors, since I don't keep raw quartz crystals.
Things that people need in bulk too, if you have a good price and good stock, people might choose you for ordinary items since it'd be easier than constantly searching each station
Good luck with your shop!