Chapter 211 - 211: Officer Jenny’s Visit
After Haunter's evolution, his overall strength increased significantly, and along with the improvement in his potential, he was no longer behind any of Cain's other Pokémon.
Moreover, after evolving, Haunter had already developed his Poison-type talent. It was just a matter of teaching him a few Poison-type moves for this talent to fully manifest.
Gastly's evolution into Haunter not only meant an improvement in his physical condition but also in his form. For instance, with the two hands he now possessed, Haunter could learn more physical moves, like Poison Jab.
This move, which is both Poison-type and physical, would allow Haunter to have a better balance in his attacks.
The moves Cain had bought on the black market had not yet been used, as the timing hadn't been right.
For a Pokémon to learn moves using TMs, a learning device is needed. There are many learning devices at Pokémon Centers, but if Cain were to use them with all the TMs he owns, it would undoubtedly attract attention.
Also, although the Pokémon Center learning devices require a League ID card to be swiped to use them, and it is not explicitly stated that they register data, only a fool would trust that the League doesn't keep that information in its central database.
Although Cain's system can block data registration on the learning devices, using them in front of the Pokémon Center staff without displaying data would be too suspicious.
For this reason, Cain decided not to use the TMs at the Pokémon Center. Instead, he preferred to pay on the black market for his Pokémon to learn the moves.