I wouldn't have blood parrots as never sell fish. They are very interactive fish, a lot of people treat them like pet dogs. Common plecos definitely should never be sold, how many people have the space for a two foot long monster catfish? Even those who do, probably arent going to be keeping common plecos.
The list for fish not to stock could be endless. I think it would be better to focus on some fish that should be in stock. Some i would have on my list, in no particular order:
1. Cherry barbs (not longfin) for some reason it is becoming impossible to find the normal varieties, everyone is keeping the longfinned ones.
2. Kribensis (another "common" fish that for some reason is becoming harder to find)
3. Leopard danio (not longfin)
4. Kuhli loaches
5. Cherry shrimp (red, yellow, blue, a lot of places only have the red)
6. Amano shrimp
7. Neon tetras (duh)
8. Black neon tetras
9. Cardinal tetras
10. Bolivian rams
11. Electric blue cichlids (Jack dempsey or acaras)
12. White cloud minnows (regular and golden varieties)
13. Dojo loaches (golden and regular)
14. African butterfly fish (cool oddball, doesnt get very big, not too difficult to take care of)
15. Spotted raphael catfish (freaking invincible, able to withstand new hobbyist abuse, stays smaller than the striped)
16. Salt and pepper corydoras (small, simple easy, cooler than the bronze in my opinion)
17. Pygmy corydoras (a very cool little fish, would probably fly off the shelves. There are two species, i would go with pygmaeus)
18. Livebearers (guppies, platies, swordtails, might skip on the mollies)
19. Clown killifish (great little topwater fish)
20. Ancistrus plecos (great alternative to the common pleco, some of them look really amazing, depending on what species you have access too.)
21. Celestial pearl danios (everyone loves them)
22. Midsize gouramis (pearls, snakeskins)
23. Dwarf gouramis (common dwarf, honey, flames, would skip on the chocolates)
24. Rainbowfish (could probably go with whatever you have access too, none of them get huge and most are decently hardy)
25. Ghost catfish (look freaking cool, they are somewhat sensitive though and need to be kept in schools)
I could go on from there, but those are the top 25 i would be looking at. Obviously you could go on and add many other fish like various other danios, cories, rasboras etc, but i think that would be a good petco starter pack. More importantly, those fish sell well and by keeping the normal instead of longfin varieties of certain species you would be fulfilling a desire that seems to be growing as more stores only carry the longfinned ones.