By closing Flippa account without paid their success fee is not legal. they might be block your credit card by contacting your cc company.
Sell genuine stuffs.
this is not HF...
Credit card will not be used anywhere. You just sell your website/domain and take accept the money to your paypal from the winning buyer. Deliver the product. Thats it. Nothing is illegal in that. If you don't deliver the website/domain, then he may chargeback by emailing to paypal. So you can't do any scam with this. But yes, if you don't pay Flippa fees, then there will be no problem.