Bitcoin on testnet is valuable for people who need it but in reality it has zero monetary value and no one trade it. Why do you have to trade it using your fiats or any other cryptocurrency, it means you will pay a kind of cost to get some bitcoin testnet?

It is a trap for people who don't know what is bitcoin testnet. They will think they have a very good deal to buy cheap bitcoin than on the crypto exchanges.

Dogecoin is a low value coin but it has somewhat monetary value, not zero monetary value like bitcoin testnet.