Genuine question, but what harm does a counterfeit TM pose? Wouldn't it just not work?
All depends on the counterfeit!
Some counterfeits are produced by genuine sellers who know what they're doing, and through a TR Scanner you can easily tell they're clean. No adverse effects on a Pokémon. This encompasses maybe 10% of all counterfeits.
The next most likely are the ones that just straight up don't function. You'll load them into a TR Scanner and they'll come up blank. About 10-15% of them fall here.
Then, we have what I call the oddities. TMs that are very clearly illegal for League Challenges, but aren't explicitly adverse for the Pokémon. Some examples I've found:
An Outrage TM that had a 75% chance to confuse, not guaranteed
A Vine Whip TM that had a chance to lower the opponent's Special Defense by 1 stage (not sure what the goal there was)
A Fire Spin TM that had a chance each turn to inflict BURN
and the list goes on.
Finally, the malware. TMs can either intentionally or unintentionally cause minor to severe damage to Pokémon, both temporary and chronic.
In the case of negligent malware, Silph Co., for instance, had an issue with the TM Counter about 25 years ago, where mons who thought they had successfully landed a counter would be stuck still, and some even suffered seizures. I know someone with a disabled Chansey who had to deal with that.
As for predatory malware is compatibility malware. Hyper Beam should NEVER be taught to a Weedle, their bodies cannot handle the stress. I have personally, recently, seen one's horn severely damaged when it was told to use it.
Finally, there's the outright evil. Malicious malware, like the one created by Team Plasma about 15 years ago, causes the Pokémon to be incapable of developing Friendship... which, in turn, isolated them from not only other trainers, but Pokémon as well.
In short: if you're going to go second market, be safe, get a TR Scanner or two, and by god, do not get anything from Silph Co.