All 8 of my bettas have come from either PETCO or Walmart.
My experience is that unless the fish JUST came in, they arent going to be in prime shape. However that doesn't mean that the fish is gonna die in a few days.
These are my general guidelines when I look at bettas:
Active and responsive when I pick up the cup
No fungus like growths
No Salt like spots
NOTE: all my petco bettas had extreamly high ammonia levels and had ammonia burns. It took some time but all but one recovered
All my walmart fish had missing tail fins
- two had bad fin rot and the female - I dont know what happened to her but her tail fin was GONE. It looked like it was cut off. This too has healed over the past month.