We waste time if we try to sell a fake customer. There are at least three kinds of fake customers.
The most common type of fake customer is one without the budget or authority to make a purchase. Often, this is the heart of the difference between a marketing-qualified lead and a sales-qualified lead.
Sometimes we create a fake customer ourselves. We do it when we are tired, busy, or lazy by pretending that the prospect in front of us is exactly like the customer last week or last month. We ask about their needs and priorities, but don't listen. We use the same pitch we have used a dozen times before. We must respect our time and each customer enough to carefully and honestly help them see how the service can help with their specific needs.
The toughest fake customer is the one who believes in your solution so much that you are willing to overlook a poor fit. Prospective customers who are large and desperate fall into this category. Because they are large, the potential revenue is a real incentive to try and do the impossible and ignore poor fit. Because they are desperate, they grab onto your solution and try to make the square peg fit into the round hole. They may try to convince you to move ahead even when you absolutely know it can't work.
The kindest thing to do is to tell them, very politely, no. If you win the deal, you will both be miserable.