On occasion, I will get a special request while negotiating to buy a domain name. A domain owner will occasionally ask me to keep an email address active for a period of time or continue forwarding or resolving a domain name to a website for a set period of time. Depending on the length of time requested and my desire to buy the domain name, this is an easy request to fulfill.
One request I have heard several times is that the domain name is not allowed to be used in a specific manner once the sale is completed. For instance, the owner may be a large accounting firm and they require that the domain name is not used for another accounting firm. Even if this request is fairly easy to complete, it is a promise I can never make.