There are a lot of factors to take into account. A brief list of issues in no particular order would be :
Site Originality
Domain Name (Ease or remembering)
Age of website - How long is it up and running for ?
Page Rank - Gives an indication as to how many site are referring to yours!
Traffic amount
Traffic Quality (Targeted)
Repeat visitors - Do people come back or do they only need to visit once?
Design - Is it your own design and script or is it someone els's?
Content - Is it updated regularly? Is it updated by you or your visitors/members.
There are some other minor issues like google search positions and so on but if you feel you are quite on top of all these factors you are sure to have a good sale!
Always have your site appraised though, and domain appraised, be careful though because there are people out there who claim to appraise but only rip people off!
And always make a sale through an escrow service or a brokerage such as Sedo or Sitepoint etc.
Good luck with the sale if you are intending on selling!