IMO
Selling a domain name for $X,XXX is normally no easy feat. Consider these points....
1. The quality, length and rarity of the domain name
2. The age of the domain name
3. The number of extensions registered and developed
4. The number of potential end users
5. The exposure and visibility of the domain for sale
A good quality, short, rare, aged domain name that is taken in many extensions, with a high number of potential end users that has high visibility and exposure has a higher likelihood of selling for a high dollar amount than a domain that does not have these elements.
Otherwise, the tips that have already been shared should be very helpful.