The reality is, once you begin negotiating, they will promise to buy your domain immediately if it appraises to the price you're asking. Sounds reasonable, but their only goal is to steer you to an appraisal service they either own or profit from in some way. Once you buy the appraisal, you'll never hear from this 'buyer' again.
I'm posting this today to let you know that this appraisal scam seems to be running rampant recently, under many variations. Please be careful not to fall into this trap. Never, ever, buy an appraisal for a domain name to satisfy a potential buyer. In all my years of selling domains, I have NEVER been asked for an appraisal from a legitimate prospect, but hundreds of times from scammers.
The one that seems to have revived recently is the one mentioned in this post:
Again, there are many variations, and they're out in droves. Just a heads-up.