I also just started
vending at one location, switched from old drop shelves to ams outsider. I stocked it the first time with 4 different dry towels. The cheap cleanrite microfiber has sold best, also got a waffle weave one, more expensive, and selling less. And a shamwow type chamois which is also selling mediocre. The best is typically the throw away paper
kleenrite truck towel. Makes less but a fast nickel is better than a slow dime.
I have 3 different glass cleaners a sprayway foaming can, mothers revision, and throw away glass wipe, again the cheap throwaway sells twice as many at least.
Armor all is always a good bet and I currently have 4 varieties. Extreme tire and wheel cleaner, 4oz armorall spray, 4oz can tire shine, and armorall sponges. I would recommend the sponges for sure and possibly the 4oz plastic spray bottle for now.
Scented trees best sellers hands down for me are in order black ice, vanilla pride, rainforest mist, and true north. I also have leather, new car, and pure steel and will be adding 3 more. However these varieties I'm sure sell differently everywhere.
I have a variety of other things in the vendor but I haven't honestly had it up long enough to tell you what's selling best.
I'm also very interested in what everyone else sells the most of.