The whole point of Bitcoin testnet coins are them to be worth nothing so you can test with them. You shouldn't be able to sell them, I don' think that's the point.
Unfortunately, some shitcoin managed to change this:
There is a shitty/scammy ethereum ICO thing right now giving people shares in exchange for testnet btc, this is totally screwing up the usability of testnet and also causing antisocial dickwads to waste all our time begging for testnet coins without disclosing that the only thing they want them for is to make a quick buck.
I needed a large amount (>1.5 BTC) of testnet BTC for emulating microtransactions and developing bitcoin applications.
Why can't you do this with a small amount? I trust this guy's opinion:
Real testing just needs tiny amount, the people who "need" large amounts are selling it. (technically trading it for defi testnet defi tokens which they expect to get converted into real value once that scams they are participating in go live).
It's also forcing bitcoin development to abandon testnet for testing.
I'm still hoping another reset can solve it (for a while).