An MVP is a Minimum Viable Product. Not processing an order is not viable and there's no product. The line for the Minimum Viable Product is drawn exactly at the point where customers pay for it and receive their product. It's completely market-driven and depends on the industry.
Testing interest with fake sites is wasting people's time and I don't think it's ethical. Plus, it doesn't collect very much information. To gauge interest, it's better to give something in exchange for something from them. You give me an e-mail address, I'll tell you when you can sign up (early!) You pre-order for $5 and you'll be one of the first to receive, etc. (or in the case of crowdfunding, which is a great way to test interest, you'll get the product and help make it possible.) You spread my link on Twitter, I'll bump you up in the beta queue.
The only fake credit-card acceptance I don't mind is to respond to a click to purchase before they've entered information with "sorry, this isn't available to purchase." It's honest, it doesn't waste the visitor's time and it provides a bit of information. It's still better to collect an e-mail address in exchange for useful updates, though.