Not to put too fine a point on it. But, here's what you're risking: In Texas, it's a criminal offense to buy piranhas, it's another criminal offense to resell them, and it's a third offense to sell/ship piranhas across TX state lines.
TEXAS -- Prohibited Species List.
Lampreys, Family Petromyzontidae
All species except Chestnut Lamprey, Ichthyomyzon castaneus and Southern Brook Lamprey, I. gagei
Freshwater Stingrays, Family Potamotrygonidae
All species
Arapaima, Family Osteoglossidae
Arapaima gigas
Dourados, Family Characidae, Subfamily Bryconinae
All species of genus Salminus
Rhaphiodontid Characoids, Family Characidae, Subfamily Rhaphiodontinae
All species of genera Hydrolycus and Rhaphiodon (synonymous with Cynodon)
South American Pike Characoids, Families Characidae and Ctenolucidae
All species of genera Acestrorhynchus, Ctenolucius (Boulengerella) and Luciocharax (Hydrocinus)
African Pike Characoids, Families Hepsetidae and Ichthyboridae
All species
South American Tiger Fishes, Family Erythrinidae
All species
African Tiger Fishes, Family Alestidae, Subfamily Hydrocyninae
All species
Piranhas and Pirambebas, Family Serrasalmidae, Subfamily Serrasalminae
All species
Freshwater Eels, Family Anguilliidae
All species except American Eel, Anguilla rostrata
Swamp Eels, Rice Eels or One-Gilled Eel, Family Synbranchidae
All species
Electric Eel, Family Electrophoridae
Electrophorus electricus
Carps, Family Cyprinidae
All species and hybrids of species of genera Ctenopharyngodon (Grass Carp), Mylopharyngodon (Black Carp), Aristichthys (Bighead Carp), Hypophthalmichthys (Silver Carp), Cirrhina (Mud Carp and relatives) and Thynnichthys (Sandhkol and other Thynnichthyid Carp)
Rudd and Roach, Family Cyprinidae
All species of genera Scardinius and Rutilus
Old World Breams, Family Cyprinidae
All species of genera Abramis, Blicca, Megalobrama and Parabramis
Old World Chubs, Ide, and Dace, Family Cyprinidae
All species of the genus Leuciscus
Asps and Yellowcheek, Family Cyprinidae
All species of the genera Aspius, Pseudaspius, Aspiolucius and Elopichthys
Giant Barbs, Mahseers, and Catla, Family Cyprinidae
All species of the genera Tor and Catla and the species Barbus tor (synonymous with Barbus hexagoniolepis)
Walking Catfishes, Family Clariidae
All species
Electric Catfishes, Family Malapteruridae
All species
South American Parasitic Catfishes, Family Trichomycteridae, Subfamilies Stegophilinae and Vandelliinae
All species
Whale Catfishes, Family Cetopsidae
All species
Airsac Catfishes, Family Heteropneustidae
All species of genus Heteropneustes
Pike Killifish, Family Poeciliidae
Belonesox belizanus
Marine Stonefishes, Family Synanceiidae
All species
Tilapia, Family Cichlidae
All species of genus Tilapia (including Sarotherodon and Oreochromis)
Snakeheads, Family Channidae
All species
Asian Pikeheads, Family Luciocephalidae
All species
Old World Pike-Perches, Family Percidae
All species of the genus Stizostedion except Walleye, Stizostedion vitreum and Sauger, S. canadense
Ruff, Family Percidae
All species of genus Gymnocephalus
Nile Perch, Family Centropomidae
All species of genera Lates and Luciolates
Seatrouts and Corvinas, Family Sciaenidae
All species of genus Cynoscion except Spotted Seatrout, Cynoscion nebulosus; Silver Seatrout, C. nothus and Sand Seatrout, C. arenarius
, i doubt the texas state police are gonna go knock down his door because of a few fish.