I've had one fish with marine TB--a large SW lion. He quit eating, and began to lose fins (yes, they actually would fall off). That's about 1 infected fish for about 100+ purchases. I don't think fish TB is very prevalent, and it would not be enough to prevent me from buying a fish.
IMO, while it is true that some LFS fish are not as robust, this statement is being made without any proof to back it up. All three books being cited are 20+ years old, so I doubt this statement, or the one about TB is coming from them. I have owned 5 dwarf gouramie--1 neon, 1 flame, and 3 sunsets. They've all been healthy, aggressive fish with active appetites and healthy behavior. The fish I lost were due to my error in tank mates and care, not any defect of the fish.