Just stumbled on the thread is all
There's no "exact science" to what to stock. Many people ask what I use and recommend so I move most of the stuff that I .... use and recommend.
The store I used to work at pushed a lot of different stuff than I do, and I started carrying their big movers when I first opened but found that that stuff didn't move as well.
For some unknown reason, in this area anyway, stores push "Stress Coat" like it was going out of style. It just gunks up the water. I'm sure it's good if you have an injured fish etc., (I carry Seachem's compatible product and it's better - no aloe, no scum)... but I break a lot of people of that habit - it's a waste of money, IMO.
The Melafix and stuff is pretty useless also IMO. I've got a couple of FW customers that use it, so I have it for them, but it's never on my "recommended" list. I also try to suggest real remedies for stuff like cyano instead of chemical fixes - fix the source of the problem, not just the symptom.
I do have a lot of stuff that simply doesn't move. When it does finally move, I usually try to replace it with something that does. A store only has so much shelf space - no sense filling it with "dust collectors" and when I got into selling FW fish a few years ago I had to re-learn a lot.
It's not an exact science. You have X, they want Y. You get Y, they want Z.
OH and I don't carry Power Sweeps either

But Maxijets and Hydor Flos work a lot better.
People buy junk because it's there. If it's not there, they'll buy the good stuff (unless junk is cheap down the road...) Retail is a frustrating business to say the least at times.
Poly filters, replacement cartridges, carbon, media bags, salt, frozen food, bulk filter pads, scraper blades and such move well and often. Whatever filters you sell the most of, keep enough replacement media on hand at all times because that moves well. If your distributor has a special, stock up.
Go to trade shows if/when you can - keep up with the "latest things" even if they're junk - if you've had the chance to see them firsthand and play with them, you will know what people will be looking for and what to stock - or what to dissuade them from buying if it's not good. You can also give manufacturers feedback on their products.
Books. Lots and lots of books. When a new customer comes in looking to purchase a tank setup - that's where I bring them first. If they look at the price on the back of a book and wrinkle their nose, I know immediately if they are serious about a new hobby or not. I can't believe some people will drop hundreds or even thousands on a setup, without a $35 "owner's manual". Said books are also a good thing to have for yourself to look things up when you get hit with a question you don't know the answer to
Jenn