Buyer's remorse simply is the desire to change one's mind and cancel or change a transaction, which sets in as soon as immediately after the sale happens ("before the ink is dry") up to a few hours or days later. It's regretting our actions and wanting to set them aside - a type of "do-over."
There are a few classic reasons that this happens, and there are a few ways to mitigate, prevent, or eliminate it. It doesn't always work, but its success rate is good.
The main reason that buyer's remorse occurs (when it happens with us and something we are purchasing, or when our client or customer tells us they want to cancel the transaction they have concluded with us) is that the sale seemed to happen too quickly. The buyer (us when we are buying something or our client when they are purchasing from us, depending on the situation) felt rushed, wasn't really confident that the proposed solution or product would fulfill their needs or expectations, or they couldn't justify the price but went ahead anyway.
Often the emotional energy of the salesperson seems so persuasive that the buyer goes ahead and agrees to purchase even though they were not fully committed to doing so when the conversation started. When people are ready to make a decision, but maybe aren't sure of the color, style, model, or other aspects of their final choice, they still are ready to buy and are more likely to keep what they purchased than when they feel talked into doing something.
If the value proposition seems reasonable to someone, they likely won't seek to change or cancel their decision over price - unless other aspects of the sale aren't quite right also. Essentially, when someone isn't ready to purchase and does so anyway, remorse can set in quickly.
The argument can be made that someone would never purchase something for which they weren't at least somewhat interested in having, and that is partially true. However, the timing of that decision, the amount of the purchase, and the details of it may be quite different than what they really wanted to have happen.
When we are working with someone who needs a simple kitchen remodel with a limited budget to invest in the project, and we talk them into a much larger decision with more extensive changes or more expensive finished or appliances, it may sound great at the time they are hearing it from us - caught up in the moment and sensing our enthusiasm for the solution. They may even reluctantly agree to it - knowing that it is more than they had decided to do. This doesn't mean that it's the correct choice for them or that they are going to allow the agreement to move forward without rescinding it.
If we know what the client wants, what will serve their needs, what their budget will accommodate, who all of the decision makers are who need to approve the decision, and they like us and consider what we have presented to them as being a reasonable solution for their concerns, there is a real good chance that a sale will happen and that it will not cancel. In this case, they would have been ready emotionally to make this decision, and they would have understood that we presented a reasonable proposal that aligned with their expectations. That's how to make a sale and avoid buyer's remorse.