You're going to need to offer proof to them that you actually ordered a good or service and didn't receive it. It will depend on what the seller says in response to the chargeback. Having been on the seller side of these and winning most of them, it comes down to how far the buyer is willing to go vs. how far the seller is willing to go. If you tell them you were buying BTC, you're going to lose. It's against the T&C. I would try and figure out a service that you could have ordered that would be hard for the seller to prove that they didn't provide to you.