This is a concept that was taught during my door-to-door selling days with The Southwestern Company. A similar example to this is when you "oversell" something and the prospect loses interest.
To understand what the buying line is you must understand the steps that precede it. Let's say you are doing a demo and it's looking like the prospect is responding positively and/or has even been very clear about their interest -- like you have asked the right questions and have received solid responses -- it's now safe to say that it seems like the prospect has exceeded the buying line.
In a b2c sale like door-to-door or insurance sales, this might be the time where you begin your closing process. Something like asking for payment methods, etc. In a b2b sale, this is the point where you want to start asking for and suggesting next steps. Next steps in b2b could consist of asking for introductions to other team members/decisions makers and setting up a next meeting.
Be Careful, Be Aware
Sometimes we exceed the buying line, fail to leverage it as an opportunity to advance the sale, and risk the buyer actually failing below the buying line. This can happen if you demo a lot longer than you should and lose the prospect.
You want to get the prospect to exceed the buying line and identify that they have reached this point by asking the right questions and observing their reactions and behavior.
Here are a few questions to ask to be able to better understand where the prospect is:
-"Does this look like something that would be helpful?"
-"Based on what you have seen today, does (your product's name) look like it can potentially help you (refer to your product's value prop -- might be "increase productivity or understand your team's strengths and weaknesses)?
Rinse and Repeat
The best way to become really good at understanding when you have exceeded the buying line is by experience. You should have as many sales conversations as possible!
I promise that you will become very good at understanding your prospects and when they react/respond in positive/negative ways.