It's all very well for you to say this as a seller but turn it around to where you become the buyer and I am not so sure you'd feel the same.
Imagine your domain name purchasing budget for this month is £20,000. A seller advertises a domain name for sale and you agree by email to purchase it for £15,000. Later on you see another domain name for sale. Unfortunately that domain name eventually sells for more than your remaining £5,000 but less than your original budget of £20,000. You have to pass on the second domain name because you've already commited to buying the first for £15,000. Shortly after this, the seller of the first domain name change his/her mind. Not only have you lost the first domain name but you've also lost the second because when that was for sale you didn't have enough in your budget. Tough luck you may say but I think that it is unprofessional to change your mind in this way. It is quite conceivable that domain purchasers are given budgets so for sellers to give buyers the runaround once a sale has been agreed isn't responsible or good business.
I've experienced someone changing their mind on a sale. In hindsight I am actually quite pleased that I didn't purchase the domain name but at the time I was annoyed. Reputation is very important in this industry. If you mess a buyer about, they'll probably tell others. Don't be surprised when some people decide not to do business with you as a result.